Sera from 44 patients with a well-documented diagnosis of chronic active hepatitis (CAH) were analysed for antibodies to hepatitis A virus (anti-HAV), hepatitis B surface antigen (HBsAg) and anti-HBs, e-antigen (HBeAg) and anti-HBe, as well as antibodies to hepatitis B core antigen (anti-HBc). Twenty-two patients had serologic evidence of hepatitis A infection. The frequency of anti-HAV was low in patients under 50 years of age (21%) but high among older patients (72%). There was, however, no significant difference between patients and age-matched controls regarding the prevalence of anti-HAV in serum. Markers for hepatitis B virus were found in 10 patients or 23% as compared with about 10% in Swedish blood donors. The results indicate that hepatitis A virus is of little importance in the pathogenesis of CAH and confirm the association between hepatitis B virus and development of chronic active hepatitis.