Electromyograms were recorded on 15 patients with rheumatoid arthritis (12 cases of classical rheumatoid arthritis and 3 cases of definite rheumatoid arthritis) and 5 cases of healthy controls. The muscles selected were the biceps brachii, opponens pollicis and abductor pollieis brevis, and following results were obtained: 1) The amplitude and mean potential duration in rheumatoid arthritis were less than controls, the differences being statistically significant. 2) Electromyographic evidence of myogenic atrophy
was demonstrated in the small hand muscles and proximal muscles. 3) Myogenic atrophy due to steroid therapy was considered in proximal muscles, while the small hand muscles were affected more rarely. 4) Myogenic atrophy in stage III-IV were less than those in stage I-II, but in the m. opponens pollicis there were little differences. 5) Myogenic atrophy in inactive rheumatoid arthritis were significantly less than those in active, but in the m. opponens pollicis no marked changes were observed. 6) Myogenic atrophy of the m. opponens pollicis were more than that of m. abductor pollicis brevis.