The clinical features of opposable triphalangeal thumb were examined in 15 patients (23 hands). Triphalangeal thumb appeared as an isolated anomaly in 16 hands and in combination with cleft hand in 7 hands. Polydactylous changes in the distal phalanx were frequently observed in these hands and in the opposite thumbs in unilaterally affected cases. Opposable triphalangeal thumb seemed to have formed as a result of incomplete fusion of the duplicated distal phalanges or fusion of the thumb and index finger. Twelve hands in ten patients were treated surgically and followed for an average of 6.5 year. Nine hands were treated by removal of the accessory phalanx, and good results were obtained in all except one. Removal of the accessory phalanx seems to be a simple and reliable treatment for opposable triphalangeal thumb in young children.