A stress fracture of the diaphyseal ulna occurred in a 19-year-old female competitive table tennis player with a history of secondary amenorrhea. Stress fractures are one of more common problems occurring in vigorous sports. Stress fractures of the ulna can result from any repeated forearm flexor muscle activity. In our case stress was caused by increased athletic activity by additional training twice a day six times a week starting eighth weeks before first signs of pain and by rhythmic repeated submaximal work load in a new job additional to training. Physical training is often associated with menstrual irregularity. Hormonal factors were known for six months. Spontaneous onset of pain in women and previous history of amenorrhea should include stress fractures as differential diagnosis.