The pathogenetic ability of hepatitis B virus (HBV) antigenemia to induce IgA nephropathy was examined in 10 patients with IgA nephropathy and HBV antigenemia. They had no previous history of liver diseases and their liver function tests were normal. All were positive for hepatitis + surface antigen (HBsAg) and antibody to hepatitis B core antigen (anti-HBcAg) with high titers, thereby indicating they were persistent carriers of HBV. Immunoperoxidase studies using monospecific polyclonal antibodies revealed HBcAg and HBsAg in the nuclei and cytoplasm of glomerular mesangial cells in 8 patients. These findings suggest immune complexes involving HBcAg and HBsAg may induce IgA nephropathy in persons who carry HBV.