Hepatitis B virus (HBV) is a major cause of chronic liver disease in southern Italy. In the same area superinfection with the delta agent is endemic. To assess the prevalence of delta infection in a large population of patients with acute and chronic HBV related liver disease and to look for differential features among delta infected and uninfected subjects sera from 592 consecutive HBsAg positive patients were tested for the delta/anti-delta system by RIA. In no case was delta Ag found in serum. The prevalence of anti-delta was low in acute hepatitis (6.6%) and in asymptomatic carriers (6.4%) but raised in chronic active hepatitis with or without cirrhosis (52.3%). A decrease in frequency of anti-delta was seen in inactive cirrhosis (38.8%) and in hepatocellular carcinoma (11.9%). A younger mean age of delta-infected subjects was observed in each type of chronic liver disease. Our data confirm that delta agent superinfection is definitely associated with severe chronic active liver disease. The difference in age between anti-delta positive and negative patients suggests that delta infection accelerates the natural history of HBV related liver disease.