Disease activity of SLE was considered low according to clinical manifestations and serum C3 (121.9 mg/dl), C4 (38.5 mg/dl), CH50 (53 U/ml) and anti-DNA antibody (2.9 IU/ml) levels, although she showed overt proteinuria (P/C = 3.4). Here, C3 is linked to systemic lupus erythematosus.