CD4+CD25+ T cells have been shown to play a regulatory or suppressive role in the immune response and are possibly relevant to the pathogenesis of autoimmune diseases. In the present study, we attempted to investigate the levels of CD4+CD25+ T cells in the peripheral blood (PB) and synovial fluid (SF) of patients with rheumatoid arthritis (RA) and the effects of CD4+CD25+ T cells on the in vitro cytokine production by stimulated SF mononuclear cells (SFMC). The results showed that RA patients had similar frequencies of CD4+CD25+ T cells in PB, expressed as a percentages of the lymphocyte population, as did healthy subjects (mean ± SD: 10.52 ± 5.87% versus 11.11 ± 4.58%., respectively). But in contrast to PB, the SF of RA patients contained significantly higher levels of CD4+CD25+ T cells (17.77 ± 7.92% versus 10.52 ± 5.87%, respectively. P < 0.001). When cocultured in vitro with SFMC, CD4+CD25+ T cells purified from either PB or SF were found to exert a considerable suppressive effect on the production of cytokines including TNF-α, IFN-γ and interleukin-10 (IL-10). The percentages of inhibition of each cytokines ranged from 41.8 to 98.4% (mean, 80.0%) for TNF-α, 42.8 to 98.9% (mean, 83.2%) for IFN-γ and 59.3 to 96.6% (mean, 80.0%) for IL-10. Because both pro-inflammatory and anti-inflammatory cytokines were suppressed by CD4+CD25+ T cells, whether CD4+CD25+ T cells might play a beneficial role in the suppression of sustained inflammation in rheumatoid synovium remains to be elucidated.