diff --git a/signalfx/signalflow/computation.py b/signalfx/signalflow/computation.py
index 67d7be2..56c2a3c 100644
--- a/signalfx/signalflow/computation.py
+++ b/signalfx/signalflow/computation.py
@@ -109,14 +109,19 @@ def stream(self):
                 self._state = Computation.STATE_COMPLETED
                 continue
 
-            # Intercept metadata messages to accumulate received metadata.
-            # TODO(mpetazzoni): this can accumulate metadata without bounds if
-            # a computation has a high rate of member churn.
+            # Intercept metadata messages to accumulate received metadata...
             if isinstance(message, messages.MetadataMessage):
                 self._metadata[message.tsid] = message.properties
                 yield message
                 continue
 
+            # ...as well as expired-tsid messages to clean it up.
+            if isinstance(message, messages.ExpiredTsIdMessage):
+                if message.tsid in self._metadata:
+                    del self._metadata[message.tsid]
+                yield message
+                continue
+
             if isinstance(message, messages.InfoMessage):
                 self._process_info_message(message.message)
                 self._batch_count_detected = True
diff --git a/signalfx/signalflow/messages.py b/signalfx/signalflow/messages.py
index ac10187..3bb565f 100644
--- a/signalfx/signalflow/messages.py
+++ b/signalfx/signalflow/messages.py
@@ -19,6 +19,8 @@ def decode(mtype, payload):
             return EventMessage.decode(payload)
         if mtype == 'metadata':
             return MetadataMessage.decode(payload)
+        if mtype == 'expired-tsid':
+            return ExpiredTsIdMessage.decode(payload)
         if mtype == 'data':
             return DataMessage.decode(payload)
         if mtype == 'error':
@@ -217,6 +219,25 @@ def decode(payload):
         return MetadataMessage(payload['tsId'], payload['properties'])
 
 
+class ExpiredTsIdMessage(StreamMessage):
+    """Message informing us that an output timeseries is no longer part of the
+    computation and that we may do some cleanup of whatever internal state we
+    have tied to that output timeseries."""
+
+    def __init__(self, tsid):
+        self._tsid = tsid
+
+    @property
+    def tsid(self):
+        """The identifier of the timeseries that's no longer interesting to the
+        computation."""
+        return self._tsid
+
+    @staticmethod
+    def decode(payload):
+        return ExpiredTsIdMessage(payload['tsId'])
+
+
 class DataMessage(StreamMessage):
     """Message containing a batch of datapoints generated for a particular
     iteration."""