diff --git a/Doc/library/ctypes.rst b/Doc/library/ctypes.rst
index ae10b71d4a0..f46da85a577 100644
--- a/Doc/library/ctypes.rst
+++ b/Doc/library/ctypes.rst
@@ -1022,12 +1022,18 @@ As we can easily check, our array is sorted now::
    1 5 7 33 99
    >>>
 
-**Important note for callback functions:**
+.. note::
 
-Make sure you keep references to :func:`CFUNCTYPE` objects as long as they are
-used from C code. :mod:`ctypes` doesn't, and if you don't, they may be garbage
-collected, crashing your program when a callback is made.
+   Make sure you keep references to :func:`CFUNCTYPE` objects as long as they
+   are used from C code. :mod:`ctypes` doesn't, and if you don't, they may be
+   garbage collected, crashing your program when a callback is made.
 
+   Also, note that if the callback function is called in a thread created
+   outside of Python's control (e.g. by the foreign code that calls the
+   callback), ctypes creates a new dummy Python thread on every invocation. This
+   behavior is correct for most purposes, but it means that values stored with
+   `threading.local` will *not* survive across different callbacks, even when
+   those calls are made from the same C thread.
 
 .. _ctypes-accessing-values-exported-from-dlls: