Grip strength, ring size, duration of morning stiffness, and the number of tender joints improved significantly in 9 patients with severe rheumatoid arthritis during prolonged continuous removal of thoracic duct lymphocytes through a surgical fistula. There was no improvement in 4 subjects in whom surgery failed to establish satisfactory lymph drainage. Reinfusion of unlabeled or 51Cr-labeled autologous lymphocytes resulted in transient exacerbation of disease activity in 3 subjects. Following reinfusion, some 51Cr-labeled lymphocytes could be found in the inflamed synovium and synovial fluid by autoradiography, and radioactivity was detected over the joints by surface counting of gamma radiation. Active rheumatoid arthritis recurred in all subjects at variable intervals after cessation of lymph drainage. These findings are compatible with the hypothesis that some of the lymphocytes in the thoracic duct lymph are essential for the continued activity of the inflammation associated with rheumatoid arthritis.