The metacarpophalangeal joint of the thumb has less range of motion than any other digital joint. Its stability is more important than its mobility. Thus, arthrodesis of this joint causes relatively little loss of function, provided only that the joint is fused in the correct position. The authors reviewed all MP joint fusions of the thumb which were done at their hospital between 1974 and 1985. They examined individually forty-one of the fifty patients and studied the indications, surgical techniques and difficulties, and the complications. Furthermore, they measured all angles of fusion and concluded that the optimal position is fifteen degrees of flexion and ten degrees of pronation. The use of tension band wiring facilitates the achievement and maintenance of the desired position. The stability of the fusion permits exercise immediately after the operation; this helps to avoid a tenodesis of the long extensor tendon of the thumb.