Fourteen out of 63 children with primary glomerular disease had membranous nephropathy (MN) that was not associated with systemic lupus erythematosus. Hepatitis B surface antigen (HBsAg) was detected in the sera of all 13 patients tested, but was found in only 6.25% of their parents, in 28.57% of their siblings, and in 18.8% of the children with other types of primary glomerular disease. These findings strongly suggest that MN in children in Taiwan is closely related to hepatitis B virus (HBV) infection, and that, in most, if not all, instances it is due to horizontal infection rather than vertical transmission. However, the antigen antibody system involved remains to be identified. In this study, 4 out of 12 cases also showed abnormal elevation of SGOT and SGPT. Liver biopsies obtained from 9 patients demonstrated chronic persistent hepatitis in 3 cases, focal necrosis in 2, and minimal change or normal histology in 4. Although both the liver and kidney diseases appeared to be relatively benign during a limited period of observation, the long-term influence of the diseases and their final outcomes remain to be clarified. With the strong association of HBs antigenemia and MN in children, and the high incidence of HBsAg-positive chronic hepatitis in MN, an investigation of liver function and HBsAg carriage in patients with MN and a study of renal function in HBsAg carriers are highly recommended.