diff --git a/CHANGELOG.md b/CHANGELOG.md
index 992dcff6..172072cc 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 ## Unreleased
 
+- Fix generate the idle connection list. (#920)
 - Handle `SSLError` exception. (#918)
 
 ## 1.0.5 (March 27th, 2024)
diff --git a/httpcore/_async/connection_pool.py b/httpcore/_async/connection_pool.py
index 214dfc4b..da43b8dc 100644
--- a/httpcore/_async/connection_pool.py
+++ b/httpcore/_async/connection_pool.py
@@ -251,7 +251,13 @@ def _assign_requests_to_connections(self) -> List[AsyncConnectionInterface]:
                 closing_connections.append(connection)
             elif (
                 connection.is_idle()
-                and len([connection.is_idle() for connection in self._connections])
+                and len(
+                    [
+                        connection
+                        for connection in self._connections
+                        if connection.is_idle()
+                    ]
+                )
                 > self._max_keepalive_connections
             ):
                 # log: "closing idle connection"
diff --git a/httpcore/_sync/connection_pool.py b/httpcore/_sync/connection_pool.py
index 01bec59e..13087c01 100644
--- a/httpcore/_sync/connection_pool.py
+++ b/httpcore/_sync/connection_pool.py
@@ -251,7 +251,13 @@ def _assign_requests_to_connections(self) -> List[ConnectionInterface]:
                 closing_connections.append(connection)
             elif (
                 connection.is_idle()
-                and len([connection.is_idle() for connection in self._connections])
+                and len(
+                    [
+                        connection
+                        for connection in self._connections
+                        if connection.is_idle()
+                    ]
+                )
                 > self._max_keepalive_connections
             ):
                 # log: "closing idle connection"
diff --git a/tests/_async/test_connection_pool.py b/tests/_async/test_connection_pool.py
index 2fc27204..0dd3646d 100644
--- a/tests/_async/test_connection_pool.py
+++ b/tests/_async/test_connection_pool.py
@@ -830,3 +830,33 @@ async def trace(name, kwargs):
         "http11.response_closed.started",
         "http11.response_closed.complete",
     ]
+
+
+@pytest.mark.anyio
+async def test_connection_pool_with_idle_connections():
+    """
+    When the max_keepalive_connections is not set, we can get 10 idle connections after 10 requests,
+    if max_keepalive_connections is set to 5, we only get 5 idle connections after 10 requests.
+    """
+    network_backend = httpcore.AsyncMockBackend(
+        [
+            b"HTTP/1.1 200 OK\r\n",
+            b"Content-Type: plain/text\r\n",
+            b"Content-Length: 13\r\n",
+            b"\r\n",
+            b"Hello, world!",
+        ]
+        * 10
+    )
+
+    async with httpcore.AsyncConnectionPool(network_backend=network_backend) as pool:
+        for i in range(10):
+            async with pool.stream("GET", f"https://example{i}.com/") as response:
+                await response.aread()
+
+        assert 10 == len([c for c in pool.connections if c.is_idle()])
+        pool._max_keepalive_connections = 5
+        for i in range(10):
+            async with pool.stream("GET", f"https://example{i}.com/") as response:
+                await response.aread()
+        assert 5 == len([c for c in pool.connections if c.is_idle()])