The clinical picture produced by tenosynovitis of the extensor carpi ulnaris tendon sheath has been presented. In some instances the pain may be localized by the patient as deep in the wrist joint and the true diagnosis of tenosynovitis of the extensor carpi ulnaris tendon sheath may be overlooked. Undoubtedly, many of these cases have been diagnosed incorrectly in the past. We feel certain that the diagnosis of injury to the triangular ligament of the wrist joint has been made in many instances, when the true nature was a tenosynovitis of the extensor carpi ulnaris tendon sheath. Residual pain following Colles fracture, in some instances at least, is due to a tenosynovitis of the extensor carpi ulnaris tendon; if looked for, it may be found more frequently than it has been in the past.

Hemorrhage, which probably takes place in the sheath at the time of injury, produces a non-specific inflammatory reaction in the synovial tissue. The long and markedly confined fibrous sheath, through which the extensor carpi ulnaris tendon glides, undoubtedly is a most important factor in producing the tenosynovitis, and also in keeping the condition active.

While most of the thickened synovial tissue was removed in the greater number of our cases, merely opening the fibrous tunnel throughout its entire length would probably relieve symptoms.