diff --git a/lisp/frame.el b/lisp/frame.el
index 99142b1bbd4..a169a8ec03c 100644
--- a/lisp/frame.el
+++ b/lisp/frame.el
@@ -681,15 +681,6 @@ the user during startup."
 	(nreverse frame-initial-geometry-arguments))
   (cdr param-list))
 
-(defcustom focus-follows-mouse (not (memq window-system '(mac w32)))
-  "*Non-nil if window system changes focus when you move the mouse.
-You should set this variable to tell Emacs how your window manager
-handles focus, since there is no way in general for Emacs to find out
-automatically."
-  :type 'boolean
-  :group 'frames
-  :version "20.3")
-
 (defun select-frame-set-input-focus (frame)
   "Select FRAME, raise it, and set input focus, if possible."
     (select-frame frame)