The frequency of delta virus infection, immunoserological parameters and degree of liver inflammation were studied in 190 HBsAg carriers in the Berlin area from 1976 to 1986. Delta infection was detected in one of 50 HBsAg carriers (2%) in the period from 1976 to 1980. In contrast 19 of 140 HBsAg carriers (14%) with delta infection were found in the period from 1981 to 1986. This group included 12 Germans, 5 Turks and 2 Italians. Only 6 of these subjects belonged to a so-called risk group: 3 drug addicts, 2 homosexuals and 1 hemophiliac. Eighteen of the 20 patients with HDV infection showed progressive liver disease in the follow-up period. Nine cases developed complete liver cirrhosis over five years. Variable transaminase levels and elevation of immunoglobulin G were recorded. Humoral autoimmune phenomena were rarely observed. The increasing frequency of HDV infection in the Berlin area is presumably related to tourism, national origin and membership in specific risk groups. The data in our study underline the importance of effective prophylaxis by active immunization with HBsAg vaccines.