Summary: The expression of inducible co-stimulator (ICOS) in peripheral blood T lymphocytes from the patients with systemic lupus erythematosus (SLE) and the role in the pathogenesis of SLE was investigated. By using two-color immunofluorescent staining and flow cytometric assay, the expression levels of ICOS in peripheal blood T lymphocytes from 33 patients with SLE and 16 healthy volunteers were detected. SLE diseases activity index (SLEDAI) of the patients with SLE was used to evaluate the disease activity. The correlation between the ICOS expression and SLEDAI was analyzed among the groups. The results showed that the expression levels of ICOS in T lymphocytes in active SLE group was markedly higher than those in the control and inactive SLE groups (both P<0.01). There was no significant difference in the expression levels of ICOS between the inactive SLE and the control groups (P>0.05). In active SLE and inactive SLE groups, positive linear correlation was found between the levels of the ICOS expression in T lymphocytes and SLEDAI (r=0.711, P＝0.001; r=0.561, P＝0.03). It was suggested that the expression of ICOS in peripheral blood T lymphocytes from the patients with active SLE was up-regulated and and ICOS might be related to the pathogenesis of SLE.