In 1984 intravenous drug users accounted for 25% of all cases of acute hepatitis B and for 32% of all cases of non-A/non-B hepatitis recorded in the Canton of Zürich. Drug addicts also represented an important risk group for hepatitis A, which occurred in small epidemics. Among 141 "healthy" drug users, 10 (7%) individuals showed signs of ongoing hepatitis B virus (HBV-) infection, 26 (19%) individuals had the finding "anti-HBc-alone" (unresolved HBV-infection) and another 69 (49%) individuals were HBV-immune. 30%, 23% and 16% of the three above groups of individuals with HBV-markers were also anti-delta virus positive. Of the 141 "healthy" drug users 23 (16%) showed signs of an ongoing hepatitis A virus (HAV-) infection and 63 (45%) individuals were HAV-immune. Anti-HTLV III/LAV was detected in 69 (49%) of the i.v. drug users, with a higher prevalence (60%) in females than in males and with a higher prevalence in individuals with HBV-markers (50-62%) than in those without such markers (19%). Thus the i.v. drug users are at high risk for infections with hepatitis viruses A, B, D and those responsible for non-A/non-B hepatitis, as well as with the AIDS-associated virus HTLV III/LAV. Preventive measures should include a reduction of needle sharing and of promiscuity, which is abundantly practised by i.v. drug users.