Periarteritis nodosa was observed in three of 266 persistent hepatitis B antigen (HBsAg) carriers undergoing long-term hemodialysis; no cases of necrotizing vasculitis occurred among 384 other patients undergoing dialysis having either no or transient antigenemia. Circulating e antigen, but no e antibody, was found in two of these three patients. The serum level of the third component of complement (C3) was normal in two patients and low in the third. Circulating immune complexes were demonstrated in all three patients, using polyethylene-glycol (PEG) precipitation, PEG-C4, and solid phase C1q tests. HBsAg and anti-hepatitis B antibody (HBsAb) were identified in the PEG precipitates using radioimmunoassay and electron microscopy technics. Direct immunofluorescence performed on a muscle biopsy specimen from one patient was positive for HBsAg, but not for immunoglobulin G (IgG), immunoglobulin M (IgM), C3 or C1q. These data support the hypothesis that circulating immune complexes involving HBsAg may be involved in the pathogenesis of periarteritis nodosa.