To investigate the role of hepatitis B virus in the pathogenesis of membranous nephropathy (MN) in China, we studied 67 MN patients (excluding lupas nephritis V) with immunoperoxidase technique. In 46 (68.2%) of the 67 MN patients, HBV antigens (HBsAg, HBcAg and HBeAg) were detected in their glomeruli, and HBsAg, the dominant antigen was detected in 45 patients (67.2%). HBV-Ag was detected less frequently in the older MN group (33.3%) (> 50 years old) than in the two younger MN groups (73.1%, 75%) (< 30 years old, 30-50 years old) (P < 0.05). 30 randomly selected MsPGN patients were studied as controls. HBV antigens were detected in only 3 out of the 30 MsPGN patients. The HBV antigen positivity rate of the MN group was significantly higher than that of the MsPGN group (P < 0.01). The results suggested that HBV may be the main cause of MN in China, especially in those younger than 50 years, but not of MsPGN. HBsAg containing immune complexs (IC) may play an important role in the pathogenesis of HBV-MN.