I don't know how you called this but since the assert fired you probably called it from the thread. See this quite recent thread regarding that http://www.codeguru.com/forum/showthread.php?t=516854

What you can do instead is to let the dialog code catch the message that the radio button sends when being selected and set a member variable to specific value. Probably an enum like Waist, Shoulder ... and so on. That member variable can safely be accessed by the thread if protected by for instance a CriticalSection see http://msdn.microsoft.com/en-us/libr...(v=VS.85).aspx