Chronic proliferative synovitis of 27 metacarpophalangeal joints in 16 horses is described. The diagnosis was based on a history of lameness and, or, poor performance, pain on flexion of the metacarpophalangeal joint, the response to intra-articular anaesthesia, and plain and contrast radiography. Radiographic findings included concavity of the distal dorsal metacarpus proximal to the sagittal ridge, and an increase in size of the synovial tissue adjacent to the proximal, dorsal attachment of the joint capsule. Mineralisation of the synovial tissue was present in some joints, and chip fractures from the dorsal aspect of the proximal phalanx were also occasionally seen. Treatment by arthroscopic resection of the tissue gave excellent results.