16 patients developed locking of the M.P. joint of the thumb as the result of a hyperextension injury. Four were successfully treated by manipulation under an intra-articular injection of a local anaesthetic, while the remaining twelve required open reduction. All were examined 4 to 48 months after treatment (average 12 months). The torn palmar plate and the accessory collateral ligament, which acted as a constrictive band, were trapped on the prominence of the radial condyle of the metacarpal. Four of the twelve surgically-treated patients had a deformity which we believe was congential: a transverse groove over the radial condyle of the metacarpal. No patients had a recurrence of the locked joint.