CD4(+)CD25(+) regulatory T cells play an important role in preventing autoimmunity. We investigated the presence of CD4(+)CD25(+) regulatory T cells in the peripheral blood of patients with rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), and systemic sclerosis (SSc), using flow cytometry. The percentage of CD4(+)CD25(+) regulatory T cells was significantly decreased in RA, especially in patients with high serum levels of either CRP or MMP-3. In SSc and SLE, the percentage of CD4(+)CD25(+) regulatory T cells was higher in patients than in controls, but not significant. We also investigated the serum levels of IL-10, which influences the function of CD4(+)CD25(+) regulatory T cells and other regulatory T cells. In RA, on contrast to CD4(+)CD25(+) regulatory T cells, the serum levels of IL-10 increased in patients with higher serum levels of CRP, or MMP-3. In SLE and SSc, the serum level of IL-10 increased significantly in patients than in controls. These data thus indicated that CD4(+)CD25(+) regulatory T cells contributes to occurrence and progression of RA, and other regulatory T cells or cytokines contribute to occurrence and progression of SSc and SLE.