We have investigated hepatitis B virus (HBV) infection in systemic necrotizing vasculitis (SNV). Our approach included the detection of the viral surface antigen (HBsAg) with a radioimmunoassay employing monoclonal anti-HBs (m-RIA); in addition, HBV DNA was looked for in serum and peripheral mononuclear blood cells. Among 28 subjects with SNV, 12 were found to be positive for HBsAg with the conventional test (p-RIA) and 7 additional subjects had anti-HBc and/or anti-HBs. From the 16 HBsAg negative individuals, 9 had HBsAg epitopes identified in serum with the m-RIA test and 1 had a low amount of circulating viral DNA. In contrast, only 1 among 6 subjects with other systemic vasculitis showed a positive test for m-RIA and HBV DNA assays; this individual had acquired HIV infection through transfusions which were also probably the source of his HBV infection. HBV DNA sequences were identified in peripheral mononuclear blood cells of 9 from the 37 tested, including 2 individuals who were HBsAg positive only with m-RIA. Therefore, our study indicates a much higher rate of HBV infection in patients with polyarteritis nodosa than previously suspected.