diff --git a/docs/changes.txt b/docs/changes.txt
index 9aa667e0be..f43e582e44 100644
--- a/docs/changes.txt
+++ b/docs/changes.txt
@@ -407,6 +407,7 @@ MSW:
 
 - Fix Cygwin 1.7 build (David Gangola).
 - Allow using wxDC::DrawText() with multiline texts.
+- Fix wxBitmapButton best size determination broken in 2.9.1.
 
 All (GUI):
 
diff --git a/src/msw/bmpbuttn.cpp b/src/msw/bmpbuttn.cpp
index aea2db8f61..217b1aec63 100644
--- a/src/msw/bmpbuttn.cpp
+++ b/src/msw/bmpbuttn.cpp
@@ -136,6 +136,13 @@ bool wxBitmapButton::Create(wxWindow *parent,
 
     SetBitmapLabel(bitmap);
 
+    if ( !size.IsFullySpecified() )
+    {
+        // As our bitmap has just changed, our best size has changed as well so
+        // reset the initial size using the new value.
+        SetInitialSize(size);
+    }
+
     return true;
 }