diff --git a/stubs/twisted/internet/defer.pyi b/stubs/twisted/internet/defer.pyi
--- a/stubs/twisted/internet/defer.pyi
+++ b/stubs/twisted/internet/defer.pyi
@@ -122,3 +122,5 @@ class AlreadyTryingToLockError(Exception
 class DeferredFilesystemLock(lockfile.FilesystemLock):
     def __init__(self, name: Any, scheduler: Optional[Any] = ...) -> None: ...
     def deferUntilLocked(self, timeout: Optional[Any] = ...): ...
+
+def setDebugging(yes: bool) -> None: ...