The histopathology of hepatitis delta virus disease was studied in carriers of HBsAg with chronic hepatitis delta antigen-positive hepatitis and in serial biopsies of patients with acute hepatitis delta virus hepatitis that progressed to chronicity. There was no histologic feature distinctive of hepatitis delta virus from other types of viral hepatitis. Biopsy specimens of patients with chronic disease exhibited portal and periportal inflammation with piecemeal necrosis, conforming to a picture of aggressive hepatitis often accompanied by cirrhosis. Characteristic was a marked intralobular infiltration by mononuclear cells and a degenerative eosinophilic change of the hepatocytic cytoplasms conducive to the formation of acidophilic bodies. Liver specimens from patients with hepatitis delta virus hepatitis exhibited aspects of focal, confluent and bridging necrosis. The disease progressed to chronicity irrespective of the original histological features. The expression of intrahepatic hepatitis delta antigen was reduced in the phase of the acute hepatitis but increased in parallel with the development of chronic active liver disease. In late-stage cirrhosis, expression of hepatitis delta antigen was usually low.