The authors present a classification for polydactyly of the upper and lower limb based on a follow-up examination of 177 patients with 336 polydactylies. The malformations are described in longitudinal and transverse directions. In the transverse axis the affected rays are designated with Roman numbers from I to V. The longitudinal axis from distal to proximal is used to differentiate the rays into ten types according to their duplication assessed both anatomically and radiologically. Triphalangism, rudimentary forms, and trifid rays can also be described by this system, so they can be correlated. Evaluation of the patient group according to this classification shows that the marginal rays of hand and feet are most affected. In the longitudinal axis the metacarpo- metatarsal joint type predominates in the upper and lower limb. Using this classification it seems possible to relate different forms of polydactyly to each other and to compare therapeutic procedures and their results.