The incidence and pathogenetic role of hepatitis B surface antigen (HBsAg) were evaluated in patients with IgA nephropathy. Among 130 consecutive patients with IgA nephropathy, HBs antigenemia was detected in 4 patients (3.1%). Serum antibody to hepatitis B core antigen was positive in these 4 patients indicating that they were persistent carriers of hepatitis B virus. Serum hepatitis B e antigen (HBeAg) was detected in 1 patient, and antibody to HBeAg was positive in the other 3 patients. The incidence of HBs antigenemia was not significantly higher than the 2.0% of the general population. An immunofluorescent study in the renal tissues from the 4 IgA-nephritic patients with HBs antigenemia did not demonstrate HBsAg or HBeAg in the glomeruli. These findings suggest that HBsAg appears to play no role in the pathogenesis of IgA nephropathy.