Seventeen male patients with a fourth metacarpal fracture and fifth carpometacarpal joint injury were treated. Distinct patterns of injury were recognized and in each case fell into one of three categories. Type IA lesions were characterized by subluxation or dislocation of the fifth carpometacarpal joint without hamate avulsion fracture. Type IB lesions were identical to type IA lesions except for the appearance of a small dorsal rim hamate avulsion fracture. Type II lesions were distinguished by subluxation or dislocation of the fifth carpometacarpal joint and comminution of the dorsal hamate rim. Finally, type III lesions exhibited coronal splitting of the hamate.