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Objectives: To evaluate the roles of complement activation products C3d and C4d binding to lymphocytes in the diagnosis of systemic lupus erythematosus (SLE) in a Chinese cohort of patients., Methods: 96 patients with SLE, 44 patients with other autoimmune disease and 40 healthy control individuals were enrolled in this study. The levels of C3d and C4d binding to peripheral CD4+ T and CD19+ B lymphocyts (designated as T-C3d, T-C4d, B-C3d, B-C4d ) was assessed by flow cytometry. The diagnostic values of these biomarkers were determined by receiver-operator characteristic analysis., Results: The levels of T-C3d, T-C4d, B-C3d, B-C4d were significantly higher in SLE patients than patients with other disease and healthy controls (p<0.01). As diagnostic tools, T-C4d and B-C4d were 61.1% sensitive/94.3% specific and 63.9% sensitive/94.3% specific in differentiating SLE patients from patients with other disease and healthy controls, respectively. T-C4d and B-C4d were significantly associated with SLE disease activity as measured by the SLE disease activity index (SLEDAI) (p<0.001), low serum C3 (p<0.001), low serum C4 (p=0.006), anti-dsDNA (IIF) (p=0.001), and anti-dsDNA (ELISA) (p=0.001)., Conclusions: Complement activation products C3d and C4d binding to lymphocytes can reflect the disease activity of SLE and can be used as biomarkers for SLE.