Volar dislocation of the base of the fifth metacarpal bone is an unusual event. When it occurs, damage to the deep branch of the ulnar nerve may be expected because of its close relation to the carpometacarpal joint. A case of neuropathy of the deep ulnar motor branch due to such a dislocation is reported. Mild wasting of the first dorsal interosseous muscle and a severe loss of adduction strength of the thumb was observed 7 weeks after reduction of the dislocation. Complete functional recovery occurred 4 months after the initial injury.