Preaxial polydactyly is a common congenital hand anomaly. Here in 138 patients with 158 duplicated thumbs, a new classification of this anomaly was done from analysis of radiographs and operation records. Initially all the duplicated thumbs were arranged into from Type I to VII on the basis of radiological extent of bifurcation. Appendage type was noted as a separate entity. Type IV was found the most common and had 53 thumbs (33.6%) involved. But findings on surgery indicated that the exact level of bifurcation varied in each type, and Type VII became three times as frequent as previously noted from x-ray examination at the time of surgery. Radiologically encountered duplications at IP (Type II), MP (Type IV) and CM (Type VI) joints were specifically classified into five groups from A to E on the basis of the structural changes detected on surgery. Details in each group were discussed.