In the last 6 years, I have seen 29 patients (37 hands) who had an additional phalanx between the two other phalanges of their thumbs. These triphalangeal thumbs can be divided into opposable and nonopposable thumbs. The epiphyseal plate of the first metacarpal is at the proximal end in the opposable triphalangeal thumb, at the distal end in the nonopposable triphalangeal thumb, at both the distal and proximal ends in the complicated split hand, and at the proximal end in the complicated preaxial polydactyly. Treatment of these cases is described, as well as theories about the origin of the deformity.