OBJECTIVE: To evaluate the clinical significance of telomerase activity and telomere length in T and B lymphocytes from patients with systemic lupus erythematosus (SLE). METHODS: CD3+ (T cell) and CD19+ (B cell) lymphocytes were isolated from the peripheral blood of SLE patients and healthy controls by means of magnetic bead-coupled antibodies. SLE patients were classified as active or inactive cases according to the SLE Disease Activity Index (SLEDAI). Telomere activity of lymphocytes was measured by telomeric-repeat amplification protocol. Telomere length was measured by flow cytometry-fluorescence in situ hybridization. RESULTS: T cell telomerase activity was significantly higher in patients with both active and inactive SLE than in controls, but was lower than B cell telomerase activity in patients with active SLE, and was not correlated with SLEDAI results. B cell telomerase activity was only significantly higher than in controls in patients with active SLE, and was strongly correlated with SLEDAI. Four laboratory results, anti-dsDNA antibody titer, IgG level, C3 level, and CH50 level, were correlated with B cell telomerase activity. Telomere length in T cells was significantly shorter than in controls. In contrast, the telomere length in B cells did not differ significantly from controls. CONCLUSION: In patients with SLE, many T cells divide continuously. Their telomerase activity was higher than that in control T cells, but not so high as to prevent telomere shortening. In contrast, B cells do not divide abnormally in the inactive phase of SLE, but divide rapidly in the active phase.