An epidemiologic survey in a maintenance hemodialysis population of 300 patients was undertaken to relate the appearance of acute serositis (pericarditis, pleuritis or ascites) to HBsAg antigenemia. A significant number of incidents of serositis occurred in patients acutely or chronically infected with hepatitis B surface antigen (HBsAg) suggesting an etiologic role for the virus in the serositis of uremia. In 2 patients with both end-stage renal disease and chronic HBsAg antigenemia, immunofluorescent studies of serosal tissues showed fluorescent clusters interpreted to be HBs antigen-antibody complexes. It is concluded that an immunologic response to viremia may be one of the causes of serositis in uremia.