The epidemiology of HDV infection in Italy was assessed in a retrospective study involving 1556 HBsAg chronic carriers on their first presentation at one of the 35 Liver Units in 1987. Total anti-HD was detected in 23.4% of HBsAg carriers and was significantly more frequent in southern than in northern Italy (26.6% vs. 19.1%, p less than 0.01). Age distribution showed that 73% of the anti-HD-positive subjects, but only 56% of the anti-HD-negative subjects, were under 40 years of age (p less than 0.01). Anti-HD prevalence increased with the severity of the liver disease from 3.8% in healthy carriers to 42.5% in cirrhosis. No geographical statistical difference was found among HBsAg healthy carriers or subjects with chronic persistent hepatitis (CPH), while among patients with chronic active hepatitis (CAH) or cirrhosis anti-HD prevalence was much higher in the south (p less than 0.01). The various potential risk factors were evaluated by multiple logistic regression analysis. HDV infection was independently related to young age, residence in the south, i.v. drug abuse, a large family and household contact with an anti-HD-positive carrier. No association was found with blood transfusion or male homosexuality. These findings confirm that HDV infection is endemic in Italy, particularly in some southern areas, where intrafamily contact probably at a young age may favour the spread of the infection.