Since 1973, 15 patients, consisting of 8 boys and 7 girls, were diagnosed as having membranous nephropathy (MN). The average age at detection was 8.2 years (2-14 years). The presenting symptom was edema in 1, pyrexia in 1 and upper respiratory infection in 1 case, in the all other cases, abnormal urinalysis was detected by the school or chance urinalysis. Surface antigen of hepatitis B virus (HBs) was positive in 6 patients and negative in 9. Anti-nuclear antibody (ANA) was positive in 3 and negative in 11. In one patient, ANA was not tested. One patient who was negative for ANA was diagnosed as having SLE 4 years later. At the last follow-up, 10 patients continued to have urinary abnormalities. Among these was one case positive for HBs antigen who went into end-stage renal failure. In the other 14 patients, the serum creatinine level was below 1.4 mg/dl. All patients showed a normal mesangium or mild mesangial proliferation. The patient diagnosed as having SLE. 4 years later showed mesangial deposits at the first renal biopsy. In our experience, most patients with MN were detected by the school or chance urinalysis and six of the these had positive HBs antigen. Lupus nephritis must be ruled out in making a diagnosis of idiopathic MN.