diff --git a/docs/internals.rst b/docs/internals.rst
index b6c3d069..d22053c8 100644
--- a/docs/internals.rst
+++ b/docs/internals.rst
@@ -204,6 +204,25 @@ Stream Classes
 .. autoclass:: Stream
    :members:
 
+   .. method:: pending_bytes ()
+
+        Returns the number of bytes queued for transmission on this stream.
+        This can be used to limit the amount of data buffered in RAM by an
+        otherwise unlimited consumer.
+
+        For an accurate result, this method should be called from the Broker
+        thread, using a wrapper like:
+
+        ::
+
+            def get_pending_bytes(self, stream):
+                latch = mitogen.core.Latch()
+                self.broker.defer(
+                    lambda: latch.put(stream.pending_bytes())
+                )
+                return latch.get()
+
+
 .. currentmodule:: mitogen.fork
 
 .. autoclass:: Stream
diff --git a/mitogen/core.py b/mitogen/core.py
index dbac6f0d..c57f84cb 100644
--- a/mitogen/core.py
+++ b/mitogen/core.py
@@ -795,8 +795,9 @@ class Stream(BasicStream):
         self.sent_modules = set()
         self.construct(**kwargs)
         self._input_buf = collections.deque()
-        self._input_buf_len = 0
         self._output_buf = collections.deque()
+        self._input_buf_len = 0
+        self._output_buf_len = 0
 
     def construct(self):
         pass
@@ -866,6 +867,9 @@ class Stream(BasicStream):
         self._router._async_route(msg, self)
         return True
 
+    def pending_bytes(self):
+        return self._output_buf_len
+
     def on_transmit(self, broker):
         """Transmit buffered messages."""
         _vv and IOLOG.debug('%r.on_transmit()', self)
@@ -881,6 +885,7 @@ class Stream(BasicStream):
                 self._output_buf.appendleft(buffer(buf, written))
 
             _vv and IOLOG.debug('%r.on_transmit() -> len %d', self, written)
+            self._output_buf_len -= written
 
         if not self._output_buf:
             broker._stop_transmit(self)
@@ -890,10 +895,10 @@ class Stream(BasicStream):
         pkt = struct.pack(self.HEADER_FMT, msg.dst_id, msg.src_id,
                           msg.auth_id, msg.handle, msg.reply_to or 0,
                           len(msg.data)) + msg.data
-        was_transmitting = len(self._output_buf)
-        self._output_buf.append(pkt)
-        if not was_transmitting:
+        if not self._output_buf_len:
             self._router.broker._start_transmit(self)
+        self._output_buf.append(pkt)
+        self._output_buf_len += len(pkt)
 
     def send(self, msg):
         """Send `data` to `handle`, and tell the broker we have output. May