diff --git a/tower-balance/Cargo.toml b/tower-balance/Cargo.toml
index dacc2548..4db8f662 100644
--- a/tower-balance/Cargo.toml
+++ b/tower-balance/Cargo.toml
@@ -38,6 +38,7 @@ tower-layer = "0.1.0"
 tower-load = { version = "0.1.0", path = "../tower-load" }
 tower-service = "0.2.0"
 tower-util = "0.1.0"
+slab = "0.4"
 
 [dev-dependencies]
 tracing-fmt = { git = "https://github.com/tokio-rs/tracing.git" }
diff --git a/tower-balance/src/pool.rs b/tower-balance/src/pool/mod.rs
similarity index 78%
rename from tower-balance/src/pool.rs
rename to tower-balance/src/pool/mod.rs
index 7d1c1716..d3420f55 100644
--- a/tower-balance/src/pool.rs
+++ b/tower-balance/src/pool/mod.rs
@@ -15,12 +15,17 @@
 #![deny(missing_docs)]
 
 use super::p2c::Balance;
-use futures::{try_ready, Async, Future, Poll};
+use crate::error;
+use futures::{try_ready, Async, Future, Poll, Stream};
+use slab::Slab;
 use tower_discover::{Change, Discover};
 use tower_load::Load;
 use tower_service::Service;
 use tower_util::MakeService;
 
+#[cfg(test)]
+mod test;
+
 #[derive(Debug, Clone, Copy, Eq, PartialEq)]
 enum Level {
     /// Load is low -- remove a service instance.
@@ -41,24 +46,38 @@ where
     making: Option<MS::Future>,
     target: Target,
     load: Level,
-    services: usize,
+    services: Slab<()>,
+    died_tx: tokio_sync::mpsc::UnboundedSender<usize>,
+    died_rx: tokio_sync::mpsc::UnboundedReceiver<usize>,
     limit: Option<usize>,
 }
 
 impl<MS, Target, Request> Discover for PoolDiscoverer<MS, Target, Request>
 where
     MS: MakeService<Target, Request>,
-    // NOTE: these bounds should go away once MakeService adopts Box<dyn Error>
-    MS::MakeError: ::std::error::Error + Send + Sync + 'static,
-    MS::Error: ::std::error::Error + Send + Sync + 'static,
+    MS::MakeError: Into<error::Error>,
+    MS::Error: Into<error::Error>,
     Target: Clone,
 {
     type Key = usize;
-    type Service = MS::Service;
+    type Service = DropNotifyService<MS::Service>;
     type Error = MS::MakeError;
 
     fn poll(&mut self) -> Poll<Change<Self::Key, Self::Service>, Self::Error> {
-        if self.services == 0 && self.making.is_none() {
+        while let Async::Ready(Some(sid)) = self
+            .died_rx
+            .poll()
+            .expect("cannot be closed as we hold tx too")
+        {
+            self.services.remove(sid);
+            tracing::trace!(
+                pool.services = self.services.len(),
+                message = "removing dropped service"
+            );
+        }
+
+        if self.services.len() == 0 && self.making.is_none() {
+            let _ = try_ready!(self.maker.poll_ready());
             tracing::trace!("construct initial pool connection");
             self.making = Some(self.maker.make_service(self.target.clone()));
         }
@@ -67,14 +86,14 @@ where
             if self.making.is_none() {
                 if self
                     .limit
-                    .map(|limit| self.services >= limit)
+                    .map(|limit| self.services.len() >= limit)
                     .unwrap_or(false)
                 {
                     return Ok(Async::NotReady);
                 }
 
                 tracing::trace!(
-                    pool.services = self.services,
+                    pool.services = self.services.len(),
                     message = "decided to add service to loaded pool"
                 );
                 try_ready!(self.maker.poll_ready());
@@ -85,14 +104,19 @@ where
         }
 
         if let Some(mut fut) = self.making.take() {
-            if let Async::Ready(s) = fut.poll()? {
-                self.services += 1;
+            if let Async::Ready(svc) = fut.poll()? {
+                let id = self.services.insert(());
+                let svc = DropNotifyService {
+                    svc,
+                    id,
+                    notify: self.died_tx.clone(),
+                };
                 tracing::trace!(
-                    pool.services = self.services,
+                    pool.services = self.services.len(),
                     message = "finished creating new service"
                 );
                 self.load = Level::Normal;
-                return Ok(Async::Ready(Change::Insert(self.services, s)));
+                return Ok(Async::Ready(Change::Insert(id, svc)));
             } else {
                 self.making = Some(fut);
                 return Ok(Async::NotReady);
@@ -104,13 +128,15 @@ where
                 unreachable!("found high load but no Service being made");
             }
             Level::Normal => Ok(Async::NotReady),
-            Level::Low if self.services == 1 => Ok(Async::NotReady),
+            Level::Low if self.services.len() == 1 => Ok(Async::NotReady),
             Level::Low => {
                 self.load = Level::Normal;
-                let rm = self.services;
-                self.services -= 1;
+                // NOTE: this is a little sad -- we'd prefer to kill short-living services
+                let rm = self.services.iter().next().unwrap().0;
+                // note that we _don't_ remove from self.services here
+                // that'll happen automatically on drop
                 tracing::trace!(
-                    pool.services = self.services,
+                    pool.services = self.services.len(),
                     message = "removing service for over-provisioned pool"
                 );
                 Ok(Async::Ready(Change::Remove(rm)))
@@ -224,16 +250,19 @@ impl Builder {
         MS: MakeService<Target, Request>,
         MS::Service: Load,
         <MS::Service as Load>::Metric: std::fmt::Debug,
-        MS::MakeError: ::std::error::Error + Send + Sync + 'static,
-        MS::Error: ::std::error::Error + Send + Sync + 'static,
+        MS::MakeError: Into<error::Error>,
+        MS::Error: Into<error::Error>,
         Target: Clone,
     {
+        let (died_tx, died_rx) = tokio_sync::mpsc::unbounded_channel();
         let d = PoolDiscoverer {
             maker: make_service,
             making: None,
             target,
             load: Level::Normal,
-            services: 0,
+            services: Slab::new(),
+            died_tx,
+            died_rx,
             limit: self.limit,
         };
 
@@ -249,8 +278,8 @@ impl Builder {
 pub struct Pool<MS, Target, Request>
 where
     MS: MakeService<Target, Request>,
-    MS::MakeError: ::std::error::Error + Send + Sync + 'static,
-    MS::Error: ::std::error::Error + Send + Sync + 'static,
+    MS::MakeError: Into<error::Error>,
+    MS::Error: Into<error::Error>,
     Target: Clone,
 {
     balance: Balance<PoolDiscoverer<MS, Target, Request>, Request>,
@@ -263,8 +292,8 @@ where
     MS: MakeService<Target, Request>,
     MS::Service: Load,
     <MS::Service as Load>::Metric: std::fmt::Debug,
-    MS::MakeError: ::std::error::Error + Send + Sync + 'static,
-    MS::Error: ::std::error::Error + Send + Sync + 'static,
+    MS::MakeError: Into<error::Error>,
+    MS::Error: Into<error::Error>,
     Target: Clone,
 {
     /// Construct a new dynamically sized `Pool`.
@@ -283,8 +312,8 @@ where
     MS: MakeService<Target, Req>,
     MS::Service: Load,
     <MS::Service as Load>::Metric: std::fmt::Debug,
-    MS::MakeError: ::std::error::Error + Send + Sync + 'static,
-    MS::Error: ::std::error::Error + Send + Sync + 'static,
+    MS::MakeError: Into<error::Error>,
+    MS::Error: Into<error::Error>,
     Target: Clone,
 {
     type Response = <Balance<PoolDiscoverer<MS, Target, Req>, Req> as Service<Req>>::Response;
@@ -304,7 +333,7 @@ where
                 }
                 discover.load = Level::Low;
 
-                if discover.services > 1 {
+                if discover.services.len() > 1 {
                     // reset EWMA so we don't immediately try to remove another service
                     self.ewma = self.options.init;
                 }
@@ -334,6 +363,10 @@ where
                 // `Ready`, so we won't try to launch another service immediately.
                 // we clamp it to high though in case the # of services is limited.
                 self.ewma = self.options.high;
+
+                // we need to call balance again for PoolDiscover to realize
+                // it can make a new service
+                return self.balance.poll_ready();
             } else {
                 discover.load = Level::Normal;
             }
@@ -346,3 +379,37 @@ where
         self.balance.call(req)
     }
 }
+
+#[doc(hidden)]
+pub struct DropNotifyService<Svc> {
+    svc: Svc,
+    id: usize,
+    notify: tokio_sync::mpsc::UnboundedSender<usize>,
+}
+
+impl<Svc> Drop for DropNotifyService<Svc> {
+    fn drop(&mut self) {
+        let _ = self.notify.try_send(self.id).is_ok();
+    }
+}
+
+impl<Svc: Load> Load for DropNotifyService<Svc> {
+    type Metric = Svc::Metric;
+    fn load(&self) -> Self::Metric {
+        self.svc.load()
+    }
+}
+
+impl<Request, Svc: Service<Request>> Service<Request> for DropNotifyService<Svc> {
+    type Response = Svc::Response;
+    type Future = Svc::Future;
+    type Error = Svc::Error;
+
+    fn poll_ready(&mut self) -> Poll<(), Self::Error> {
+        self.svc.poll_ready()
+    }
+
+    fn call(&mut self, req: Request) -> Self::Future {
+        self.svc.call(req)
+    }
+}
diff --git a/tower-balance/src/pool/test.rs b/tower-balance/src/pool/test.rs
new file mode 100644
index 00000000..d73caa41
--- /dev/null
+++ b/tower-balance/src/pool/test.rs
@@ -0,0 +1,285 @@
+use futures::{Async, Future};
+use tower_load as load;
+use tower_service::Service;
+use tower_test::mock;
+
+use super::*;
+
+macro_rules! assert_fut_ready {
+    ($fut:expr, $val:expr) => {{
+        assert_fut_ready!($fut, $val, "must be ready");
+    }};
+    ($fut:expr, $val:expr, $msg:expr) => {{
+        assert_eq!(
+            $fut.poll().expect("must not fail"),
+            Async::Ready($val),
+            $msg
+        );
+    }};
+}
+
+macro_rules! assert_ready {
+    ($svc:expr) => {{
+        assert_ready!($svc, "must be ready");
+    }};
+    ($svc:expr, $msg:expr) => {{
+        assert!($svc.poll_ready().expect("must not fail").is_ready(), $msg);
+    }};
+}
+
+macro_rules! assert_fut_not_ready {
+    ($fut:expr) => {{
+        assert_fut_not_ready!($fut, "must not be ready");
+    }};
+    ($fut:expr, $msg:expr) => {{
+        assert!(!$fut.poll().expect("must not fail").is_ready(), $msg);
+    }};
+}
+
+macro_rules! assert_not_ready {
+    ($svc:expr) => {{
+        assert_not_ready!($svc, "must not be ready");
+    }};
+    ($svc:expr, $msg:expr) => {{
+        assert!(!$svc.poll_ready().expect("must not fail").is_ready(), $msg);
+    }};
+}
+
+#[test]
+fn basic() {
+    // start the pool
+    let (mock, mut handle) =
+        mock::pair::<(), load::Constant<mock::Mock<(), &'static str>, usize>>();
+    let mut pool = Builder::new().build(mock, ());
+    with_task(|| {
+        assert_not_ready!(pool);
+    });
+
+    // give the pool a backing service
+    let (svc1_m, mut svc1) = mock::pair();
+    handle
+        .next_request()
+        .unwrap()
+        .1
+        .send_response(load::Constant::new(svc1_m, 0));
+    with_task(|| {
+        assert_ready!(pool);
+    });
+
+    // send a request to the one backing service
+    let mut fut = pool.call(());
+    with_task(|| {
+        assert_fut_not_ready!(fut);
+    });
+    svc1.next_request().unwrap().1.send_response("foobar");
+    with_task(|| {
+        assert_fut_ready!(fut, "foobar");
+    });
+}
+
+#[test]
+fn high_load() {
+    // start the pool
+    let (mock, mut handle) =
+        mock::pair::<(), load::Constant<mock::Mock<(), &'static str>, usize>>();
+    let mut pool = Builder::new()
+        .urgency(1.0) // so _any_ NotReady will add a service
+        .underutilized_below(0.0) // so no Ready will remove a service
+        .max_services(Some(2))
+        .build(mock, ());
+    with_task(|| {
+        assert_not_ready!(pool);
+    });
+
+    // give the pool a backing service
+    let (svc1_m, mut svc1) = mock::pair();
+    svc1.allow(1);
+    handle
+        .next_request()
+        .unwrap()
+        .1
+        .send_response(load::Constant::new(svc1_m, 0));
+    with_task(|| {
+        assert_ready!(pool);
+    });
+
+    // make the one backing service not ready
+    let mut fut1 = pool.call(());
+
+    // if we poll_ready again, pool should notice that load is increasing
+    // since urgency == 1.0, it should immediately enter high load
+    with_task(|| {
+        assert_not_ready!(pool);
+    });
+    // it should ask the maker for another service, so we give it one
+    let (svc2_m, mut svc2) = mock::pair();
+    svc2.allow(1);
+    handle
+        .next_request()
+        .unwrap()
+        .1
+        .send_response(load::Constant::new(svc2_m, 0));
+
+    // the pool should now be ready again for one more request
+    with_task(|| {
+        assert_ready!(pool);
+    });
+    let mut fut2 = pool.call(());
+    with_task(|| {
+        assert_not_ready!(pool);
+    });
+
+    // the pool should _not_ try to add another service
+    // sicen we have max_services(2)
+    with_task(|| {
+        assert!(!handle.poll_request().unwrap().is_ready());
+    });
+
+    // let see that each service got one request
+    svc1.next_request().unwrap().1.send_response("foo");
+    svc2.next_request().unwrap().1.send_response("bar");
+    with_task(|| {
+        assert_fut_ready!(fut1, "foo");
+    });
+    with_task(|| {
+        assert_fut_ready!(fut2, "bar");
+    });
+}
+
+#[test]
+fn low_load() {
+    // start the pool
+    let (mock, mut handle) =
+        mock::pair::<(), load::Constant<mock::Mock<(), &'static str>, usize>>();
+    let mut pool = Builder::new()
+        .urgency(1.0) // so any event will change the service count
+        .build(mock, ());
+    with_task(|| {
+        assert_not_ready!(pool);
+    });
+
+    // give the pool a backing service
+    let (svc1_m, mut svc1) = mock::pair();
+    svc1.allow(1);
+    handle
+        .next_request()
+        .unwrap()
+        .1
+        .send_response(load::Constant::new(svc1_m, 0));
+    with_task(|| {
+        assert_ready!(pool);
+    });
+
+    // cycling a request should now work
+    let mut fut = pool.call(());
+    svc1.next_request().unwrap().1.send_response("foo");
+    with_task(|| {
+        assert_fut_ready!(fut, "foo");
+    });
+    // and pool should now not be ready (since svc1 isn't ready)
+    // it should immediately try to add another service
+    // which we give it
+    with_task(|| {
+        assert_not_ready!(pool);
+    });
+    let (svc2_m, mut svc2) = mock::pair();
+    svc2.allow(1);
+    handle
+        .next_request()
+        .unwrap()
+        .1
+        .send_response(load::Constant::new(svc2_m, 0));
+    // pool is now ready
+    // which (because of urgency == 1.0) should immediately cause it to drop a service
+    // it'll drop svc1, so it'll still be ready
+    with_task(|| {
+        assert_ready!(pool);
+    });
+    // and even with another ready, it won't drop svc2 since its now the only service
+    with_task(|| {
+        assert_ready!(pool);
+    });
+
+    // cycling a request should now work on svc2
+    let mut fut = pool.call(());
+    svc2.next_request().unwrap().1.send_response("foo");
+    with_task(|| {
+        assert_fut_ready!(fut, "foo");
+    });
+
+    // and again (still svc2)
+    svc2.allow(1);
+    with_task(|| {
+        assert_ready!(pool);
+    });
+    let mut fut = pool.call(());
+    svc2.next_request().unwrap().1.send_response("foo");
+    with_task(|| {
+        assert_fut_ready!(fut, "foo");
+    });
+}
+
+#[test]
+fn failing_service() {
+    // start the pool
+    let (mock, mut handle) =
+        mock::pair::<(), load::Constant<mock::Mock<(), &'static str>, usize>>();
+    let mut pool = Builder::new()
+        .urgency(1.0) // so _any_ NotReady will add a service
+        .underutilized_below(0.0) // so no Ready will remove a service
+        .build(mock, ());
+    with_task(|| {
+        assert_not_ready!(pool);
+    });
+
+    // give the pool a backing service
+    let (svc1_m, mut svc1) = mock::pair();
+    svc1.allow(1);
+    handle
+        .next_request()
+        .unwrap()
+        .1
+        .send_response(load::Constant::new(svc1_m, 0));
+    with_task(|| {
+        assert_ready!(pool);
+    });
+
+    // one request-response cycle
+    let mut fut = pool.call(());
+    svc1.next_request().unwrap().1.send_response("foo");
+    with_task(|| {
+        assert_fut_ready!(fut, "foo");
+    });
+
+    // now make svc1 fail, so it has to be removed
+    svc1.send_error("ouch");
+    // polling now should recognize the failed service,
+    // try to create a new one, and then realize the maker isn't ready
+    with_task(|| {
+        assert_not_ready!(pool);
+    });
+    // then we release another service
+    let (svc2_m, mut svc2) = mock::pair();
+    svc2.allow(1);
+    handle
+        .next_request()
+        .unwrap()
+        .1
+        .send_response(load::Constant::new(svc2_m, 0));
+
+    // the pool should now be ready again
+    with_task(|| {
+        assert_ready!(pool);
+    });
+    // and a cycle should work (and go through svc2)
+    let mut fut = pool.call(());
+    svc2.next_request().unwrap().1.send_response("bar");
+    with_task(|| {
+        assert_fut_ready!(fut, "bar");
+    });
+}
+
+fn with_task<F: FnOnce() -> U, U>(f: F) -> U {
+    use futures::future::lazy;
+    lazy(|| Ok::<_, ()>(f())).wait().unwrap()
+}