I wonder if this was due to Flash changing the way it's displayed in fullscreen. I had the same problem when Flash player was updated a while ago it's fullscreen display would not come to the foreground. When I adjusted to compensate for this it screwed up Siverlights ability to go fullscreen. The fix I had was to do the following... IntPtr hWnd = Win32.FindWindow("ShockwaveFlashFullScreen", IntPtr.Zero); if (hWnd != IntPtr.Zero) Win32.SetParent(hWnd, this.Handle); What this does is force Flash's screen into my fullscreen window but also allows Silverlight to go fullscreen without messing with it. Anyway I have no idea if this is what messed things up for Tom, but I thought I would post about it anyway.