The current classification of metacarpal synostosis is based on the extent of the synostosis. The authors propose a new classification that takes into account the shape of the metacarpal bones, the curvature of the epiphysis, and the discrepancy in length between the two bones. This classification provides better guidelines for the correction of all components of the deformity. The classification is based on the authors' observations of and experience with 36 cases of metacarpal synostosis; 13 of the deformities were surgically corrected. The I-shaped deformity, whether with distinct (type d) or fused (type f) metacarpophalangeal joints, does not require surgical correction. The U-shaped deformity has parallel epiphysis and does not require surgery unless the two metacarpals are asymmetrical in length (type a) or tightly fused (type t); in these cases, simple lengthening or widening of the space with a bone graft is sufficient. Y-shaped synostosis should be separated whether the branches are symmetrical or asymmetrical, the latter having one branch shorter than the other. Because the epiphyses are already divergent, simple separation does not effectively correct Y-shaped synostosis. The authors propose an osteotomy to isolate a trapezoidal segment of bone from the bifurcation. The isolated bone segment is then reversed in the proximal-distal direction to provide a "plateau" upon which the two distal metacarpals can be realigned. Two cases of Ys (symmetrical) synostosis were successfully treated with this technique; one case of Ya (asymmetrical) synostosis also required distraction lengthening of the shorter metacarpal to achieve an excellent result. One of the most difficult types of metacarpal synostosis to treat is k-shaped synostosis, observed only between the fourth and fifth metacarpals; in this type, the head of the short fifth metacarpal abuts the metaphysis of the fourth. Osteotomy and distraction lengthening provide predictable results for correction of this deformity. The authors suggest that k-shaped synostosis might represent a late evolution of untreated Ua synostosis.