Regulatory T cells have an important role in the control of self-reactivity, and in the pathogenesis of autoimmune inflammatory conditions. The aim of this work was to perform a quantitative and functional analysis of regulatory T cells in patients with systemic lupus erythematosus (SLE). We studied twenty-three patients with SLE (19 active, 4 inactive), and twenty-seven healthy subjects as well as fifteen patients with rheumatoid arthritis (RA). The following cell subsets were analyzed in peripheral blood mononuclear cells by flow cytometry: CD4+CD25+, CD4+CD25^bright, CD4+Foxp3+ (Treg cells), CD8+CD28− (Ts cells), CD4+IL-10+ (Tr1 cells), and CD4+TGF-β+ (Th3 cells). In addition, the in vitro suppressive activity of CD4+CD25+ lymphocytes was tested. We found no significant differences in the levels of all regulatory cell subsets studied in SLE patients compared to controls and RA patients. However, a defective regulatory function of CD4+CD25+T cells was observed in a significant fraction (31%) of patients with SLE. Our data indicate that although approximately one third of patients with SLE show an abnormal immunosuppressive function of Treg lymphocytes, their levels of the different regulatory T cell subsets in peripheral blood are not significantly different from those found in controls.