diff --git a/hbd/server/udp.py b/hbd/server/udp.py
index 831948e..4cc359e 100644
--- a/hbd/server/udp.py
+++ b/hbd/server/udp.py
@@ -398,6 +398,16 @@ def handle_datagram(msg: dict, addr, transport, ctx: dict):
                 plugin_interval = max(history[-1][0] - history[-2][0], 1)
                 host.reset_plugin_timer(plugin_name, plugin_interval * 3,
                                         _make_plugin_stale_callback(uname, ctx))
+                # Remove alert states for metrics present in the previous sample
+                # but absent now (e.g. a nagios check removed from configuration).
+                prev_keys = set(history[-2][1].keys())
+                curr_keys = set(plugin_data.keys())
+                for metric_name in prev_keys - curr_keys:
+                    metric_path = f"{plugin_name}.{metric_name}"
+                    if host.alert_states.pop(metric_path, None) is not None:
+                        eventlog(uname, "INFO", f"stale check removed: {metric_path}")
+                if (prev_keys - curr_keys) and msg_to_websockets:
+                    msg_to_websockets("host", host.stateinfo())
             else:
                 host.cancel_plugin_timer(plugin_name)