ABSTRACTWassel's classification of thumb polydactyly has some problems especially in categories IV and VII because it depends solely on X‐ray findings. In order to improve his thumb polydactyly classification, we examined histologically untreated 182 bifid thumbs from 171 patients at Branch Hospital, Nagoya University in the past eight years. Ninety‐five of these cases had proximal phalangeal separate type of polydactyly corresponding to Wassel's categories IV and VII. Forty‐five thumbs of these categories were excised from 44 patients and classified into type I (24 cases), type II (12). type III (4), and type IV (5) in accordance with Kanno's classification. In distal phalanges, normal cartilage development was observed in Kanno's type I, whereas proximally elongated cartilaginous tissue was formed in the type II. Three distal phalanges from types II and III showed depression at the ulnar side of elongated cartilage of the distal phalanx where immature chondrocytes transversely distributed. Three cases with incomplete and one case with complete joint formation were formed in elongated cartilage of the distal phalanx, and endochondral ossification developed in the central area of elongated cartilage of the distal phalanx in four cases from types III and IV. All of the cases showed only minimal mobility of their involved joints. Their tendon development did not correspond to their anatomical characteristics. Histological studies indicated that there was no distinct difference between Wassel's category IV and category VII. It was postulated that the triph