To assess the prevalence and clinical significance of delta-infection in chronic hepatitis B surface antigen carriers, we examined 326 liver biopsies from 192 retrospectively selected carriers by immunofluorescence. Delta antigen was detected in 102 specimens from 50 carriers (26.2%) with peak prevalence in patients with active cirrhosis (51.5%) and generally in close association with progressive liver disease (94%). The antigen was located in the nuclei of 2%-50% of the hepatocytes, without any disease-specific pattern of fluorescence. Patients with intrahepatic delta-antigen, however, had more severe liver disease than those without it. Histologic follow-up of 101 cases showed that the rates of worsening of the liver disease were similar in delta-antigen-positive and in delta-antigen-negative patients. It is concluded that delta-superinfection does play a role in worsening the histologic picture of hepatitis B surface antigen-positive chronic active hepatitis, possibly by liver injury induced acutely at the moment of infection.