The liver histology in infectious hepatitis or hepatitis A (HA) and serum hepatitis or hepatitis B (HB) is generally described as identical. However, the clinical separation of the two types has been a problem. Today a serological reaction based on the well documented association between hepatitis antigen and HB is of great assistance in the differential diagnosis. The present study of 165 hepatitis cases separated into hepatitis A and B by this test method indicates quantitative differences in the liver histology of the two types. Thus HB was associated with more prominent parenchymal cell damage and Kupffer cell reaction, while intrahepatic cholestasis was found in a significantly higher frequency in cases presumed to represent HA. The presence of intrahepatic cholestasis was associated with higher levels of serum bilirubin but otherwise no correlation was found between liver morphology and biochemical liver tests. The patients included a group of young intravenous amphetamine addicts with HB. No differences of importance were found histologically in addicts and other patients with hepatitis B.