Ten cases with fracture-separation of the medial humeral epicondyle occurring during arm wrestling were studied to clarify the nature of injury and the mechanism involved. Patients were all male, aged 13 to 15 years. The following results were obtained: (1) the injury often occurred in children immediately before closure of the epiphyseal plate, (2) the injury occurred only when the arm wrestler took a posture in which his center of gravity could shift easily, (3) the injury developed only when one wrestler tried to force the end of the match and the other countered that move, and (4) no lateral instability of the elbow was observed in patients with this injury. These results suggest that the injury is caused by muscular violence. When the maximally contracted muscles attaching to the medial humeral epicondyle are suddenly and passively stretched by shifting of the patient's center of gravity and by the opposing wrestler's counterattack, a shift from concentric contraction to eccentric contraction could occur and apply a large muscle force to the medial humeral epicondyle.