A case of fracture of the hook of the hamate associated with a palmar dislocation of the fifth carpometacarpal joint is described. Surgical exploration revealed that the different elements of the flexor carpi ulnaris musculotendinous unit, which includes the pisiform, the pisohamate and pisometacarpal ligaments, were displaced proximally together with the avulsed fragment of the hook of the hamate and the fifth metacarpal. These findings suggest that such an injury results from a sudden, violent contraction of the flexor carpi ulnaris against the fixed wrist. It was successfully treated by open reduction and fixation of the hook of the hamate fracture and stabilization of the carpometacarpal joint.