The distribution of CD57+ T and CD56+ T cells in patients with RA was examined. In control osteoarthritis patients, these cells exist as a minor population in the peripheral blood. Our data show that in patients with RA, CD57+ T cell levels are elevated in peripheral blood, knee joint fluid, knee synovial membrane and bone marrow (BM), compared with peripheral blood of controls. CD57+ T cells are especially high in knee joint fluid and joint-adjacent BM, while CD56+ T cells show no such increase. CD57+ T cells contain a major population of CD8+ cells and higher proportions of CD4-8- cells and gammadelta T cells than do CD57- T cells. CD57+ T cells in peripheral blood and joint fluid increase with the duration of disease. Erythrocyte sedimentation rate (ESR) is inversely correlated with the proportion of CD57+ T cells in the joint fluid. Although RA frequently occurs in patients with CD3+57+ cell leukaemia, and some CD57+ T cells are likely to be involved in the onset of RA, we suggest that CD57+ T cells may rather suppress inflammation of RA, and other cellular components (e.g. granulocytes) may govern the severity of the inflammation of RA. These CD57+ T cells are probably generated extrathymically in the adjacent BM or joint space.