Spastic thumb deformity is the result of imbalance between intrinsic and extrinsic forces acting across unstable joints. This paper presents a classification of spastic thumb deformity based on the accurate assessment of the deforming forces, outlines methods for their correction and reviews the results of our surgery. Thumb reconstruction procedures were performed in 32 patients with 33 spastic thumb deformities. All patients were assessed pre- and postoperatively using the same functional assessment system which was performed by the same team. The thumb was maintained out of the palm in 29 patients and lateral pinch was established in 26 patients. Copyright 2001 The British Society for Surgery of the Hand.