Fracture of the thumb sesamoid bones is rare. We report here a fracture of the ulnar sesamoid in a 25-year-old man caused by a fall on his outstretched hand. He complained of pain and loss of thumb motion. Physical examination revealed tenderness, swelling,, and a hematoma over the first metacarpal joint of the right thumb. Radiographs showed a fracture of the ulnar sesamoid which should not be confused with normal variants that occasionally arise from ossifications centers that fail to unite. (C) 1998 Wiley-Liss, Inc.