OBJECTIVE To investigate the expressions of C(4)D(+)CD(25)(+) T cells in the peripheral blood of patients with systemic lupus erythematosus (SLE) and to identify the relationship between the levels of CD(4)(+)CD(25)(+) T cells and the disease activity and progression of SLE. METHODS Fifty-three SLE patients were enrolled in the study. Flow-cytometric assay was employed for detection of CD(4)(+)CD(25)(+) T cells and CD(4)(+)CD(25)(bright) T cells were defined according to fluorescence intensity of CD(25) expression exceeding 100. Meanwhile, correlation analysis was performed between their expression and the scores of SLE disease active index (SLEDAI), C(3), dsDNA and antinuclear antibody titles. RESULTS The levels of peripheral blood CD(4)(+)CD(25)(+) T cells in SLE were (7.84 +/- 1.85)%, which were significantly lower than those in a group of healthy control [(9.18 +/- 2.01)%, P < 0.05]. The levels of CD(4)(+)CD(25)(+) T cells in a group of active SLE [(6.72 +/- 1.16)%] were higher than those in a group of stable SLE [(8.57 +/- 1.91)%, P < 0.01]. There was no difference between the active and stable groups of SLE in peripheral blood CD(4)(+)CD(25)(bright) T cells [(0.85 +/- 0.24)% vs (0.91 +/- 0.25)%, P = 0.686], but they were significantly lower than those in the group of healthy controls [(1.43 +/- 1.08)%, P < 0.01]. With the reduction of the SLEDAI scores in SLE patients after relevant treatment, the levels of peripheral blood CD(4)(+)CD(25)(bright) T cells did not change. No correlation was found between the levels of CD(4)(+)CD(25)(bright) T cells and SLEDAI scores, antinuclear antibody titles, dsDNA and C(3), respectively (rho = -0.188, P = 0.178; rho = -0.216, P = 0.121; rho = 0.082, P = 0.560; rho = 0.010, P = 0.944). CONCLUSIONS The reduction of CD(4)(+)CD(25)(+) T cells in peripheral blood may participate in the pathogenesis of SLE.