Synovial fluid lymphocytes and paired peripheral blood lymphocytes from 29 patients with rheumatoid arthritis (RA) were analyzed using two-dimensional flow cytometry. The lymphocytes of peripheral blood from 20 normal subjects were served as control. In the RA patients, the composition of the lymphocyte population was different in the synovial fluid and peripheral blood. Compared with the peripheral blood, the synovial fluid contained higher populations of Ia+ (Leu 4 + Leu HLA-DR+) T cells, especially Leu 2a + Leu HLA-DR+ (activated suppressor/cytotoxic) T cells, and of Leu 2a + Leu 15- (cytotoxic T) cells and Leu 3a + Leu 8- (helper T) cells. Contrary, the synovial fluid contained lower populations of Leu 2a + Leu 15+ (suppressor T) cells and Leu 3a + Leu 8+ (inducer T) cells. Compared with the lymphocytes of the normal peripheral blood, the RA patients had an elevated ratio of helper T/suppressor T cells (Th/Ts) in both the synovial fluid and peripheral blood, incidentally the former group showed striking elevation. The ratio of Th/Ts in the synovial fluid showed considerable variation, with a significant inverse correlation with the disease activity (Lansbury index). These findings suggest that patients with rheumatoid arthritis show more specific changes in their synovial fluid lymphocytes than in their peripheral blood lymphocytes.