Renal biopsy was performed in a 12-year-old girl with hematuria and proteinuria which was first detected at the age of 7, and the findings were the mesangial proliferative glomerulonephritis with IgG and C3 deposits. The routine blood examination for the biopsy disclosed the presence of the prolonged activated partial thromboplastin time and the biological false positive reaction in the syphilis test. These results led us to the further investigation, which revealed the presence of high titers of anticardiolipin antibodies. Since this girl presented no extra-renal symptoms of systemic lupus erythematosus (SLE) and had negative serologic tests for SLE, we hypothesize that her nephritis is closely related to antiphospholipid antibodies.