Five acute and chronic dislocations of the proximal interphalangeal joint were observed. All were associated with ruptures of the central slip of the extensor mechanism as shown by surgical exploration or the appearance of a boutonnière deformity after treatment. A study of the pathomechanics of this lesion in thirty fresh cadaver fingers indicated that the lesion is produced by a combination of a varus or valgus force and an anteriorly directed force on the base of the middle phalanx. Avulsion of collateral ligament, a tear of the volar plate, and rupture of the central slip are associated with the dislocation. Based on these findings and our experience with two surgically treated dislocations, procedures for the treatment of acute and chronic dislocations are proposed. The acute lesion should be treated by open reduction, fixation of the proximal interphalangeal joint in full extension with a transfixing Kirschner wire, and repair of the torn central slip, collateral ligament, and volar plate. The chronic lesion should also be reduced and fixed with a transfixing wire. However the volar plate must be excised before reduction is possible. The central slip and collateral ligaments must then be reconsitituted.