Twelve patients with recurrent dislocation of the extensor carpi ulnaris tendon were treated with repair or reconstruction of its tendon sheath, and each had a satisfactory result. We found three types of disruption of the fibro-osseous sheath. Type A: the fibro-osseous sheath ruptured ulnarly and the torn sheath lay superficial to the tendon (n=5). These were treated by reconstruction of the sheath using a piece of the extensor retinaculum. Type B: the fibro-osseous sheath ruptured radially and the torn sheath lay in the ulnar groove beneath the tendon (n=3). These were treated by direct suture of the sheath over the tendon. Type C: detachment of the periosteum from the ulnar side of the ulna in continuity with the fibro-osseous sheath formed a false pouch into which the tendon easily dislocated (n=4). These were treated by reattachment of the periosteum.