To elucidate the prognosis and the causative viral antigens of hepatitis B virus (HBV)-associated childhood membranous nephropathy (MN), the clinical course and glomerular HBV antigens were studied in 52 HBsAg carrier children with MN (40 boys, 12 girls). With Fab fragments of monoclonal antibodies, hepatitis Be antigen (HBeAg) was detected in the glomerular deposits in 41 (95%) of 43 cases but HBsAg and hepatitis B core antigen (HBcAg) in none. HBeAg was detected in sera from 43 (93%) of 46 children examined. These results suggest that HBeAg plays an important role in the development of MN in HBsAg carrier children. During the follow-up period (mean, 4 years), complete remission was found in 64% and 92% of the patients followed for one and seven years, respectively; only one child had mild renal function impairment. These findings suggest a favorable outcome of HBsAg-associated childhood MN. The patient's age, disease duration, amount of glomerular deposit, focal sclerosis and disease stage appeared to affect the clinical course. HBsAg seroconversion to HBsAg-negative occurred in seven cases, and all (100%) had quick remission in two years. In patients with persistent HBsAg carriage, serum HBeAg status alone did not correlate with remission rate and remission occurred usually before the HBeAg seroconversion to anti-HBe. These findings, together with the predominant horizontal infection in these children in contrast to the frequent vertical (perinatal) transmission from HBsAg carrier mothers in HBsAg carriers in Taiwan, suggest that factors other than HBeAg per se may also play important roles.