Abstract： Objective To investigate the association of complement C3 with clinical and serological features of patients with systemic lupus erythematosus.Methods Data was collected by the same methods in the past ten years in fifteen hospitals in Jiangsu Province and then data weres summarized for retrospective analysis.Clinical and laboratory data were selected and then analyzed by Chi-square test,Wilcoxon rank sum test and Logistic regression.Results One thousand four hundred and five patients were investigated.One thousand and forty two had low serum complement C3 level.In this case control study,hospitalization age,disease course,admission times,pleurisy,gastrointestinal involvement,general lymphadenopathy/hepatosplenomegaly,white blood cell count, haemoglobin level,platelet count, serum C-reactive protein level,serum albumin level,serum creatinine level,Urine protein quantification,anti-nuclear antibodies (ANA),anti-dsDNA antibodies, anti-Sm antibodies and SLEDAI score were possible factors associated with complement C3 reduction(P＜0.05).Logistic regression analysis showed that CRP (OR=0.396,0.254-0.617,P=0.000),ANA (OR=2.907,1.267-6.670,P=0.012),urine protein level (OR=1.702,1.043-2.779,P=0.033) and SLEDAI score (OR-0.930, 0.886-0.975,P-0.003) were correlated with complement C3 reduction.Conclusion Complement C3 level is valuable for lupus flare assessment.The complement C3 reduction is a risk factor for renal impairment.