1. The posterior interosseous nerve of the forearm is vulnerable to constriction in an aponeurotic cleft in the supinator muscle. 2. A case is presented of paresis of the nerve by an intramuscular lipoma which extended into this cleft. 3. A discussion is given of the possible relation of this feature to certain cases of occupational stress–"tennis elbow." 4. An approach is described for decompressing the nerve in the rare cases which do not respond to conservative treatment.
