In recent years chronic infection by the hepatitis delta virus (HDV) has become an important cause of chronic liver disease among drug addicts. To evaluate the influence of addiction to i.v. drugs on the course of this disease we have analyzed the clinical, histopathological, virological and evolutive features in 18 addicts and 11 non-addicts with chronic delta infection. Recent acute hepatitis D, documented as HDV superinfection, was observed in 14 addicts (77%) and in 2 non-addicts (18%) (P less than 0.02). At the time of evaluation for chronic liver disease, the frequency of symptoms, the degree of biochemical disturbances and the histopathological severity were similar in the two groups but the duration of HDV infection was probably shorter in drug addicts. HBV replication, as indicated by the presence of HBeAg and HBV-DNA in serum and HBcAg in liver, was more frequent in addicts. The amount of HDAg in liver tissue was also greater in addicts (P less than 0.005). Antibodies against the human immunodeficiency virus were detected in all of the addicts (P less than 0.001). Although most patients remained asymptomatic, significant histological worsening occurred in one half of the cases after a relatively short period of follow-up (25.1 +/- 16.3 months). The tendency to deteriorate in addicts (61% of cases) was greater than in non-addicts (36%). These observations suggest that the prognosis of chronic HDV infection is particularly poor in drug addicts in whom rapid deterioration may be related to simultaneous and inadequately controlled replication of hepatotropic viruses.