We report the results of a survey of a total of 553 intravenous drug users (IVDUs) investigated at the drug addiction out-patient unit of the Psychiatric Department of Vienna University for antibodies to human immunodeficiency virus (HIV-1) using both enzyme-linked immuno sorbent and Western blot assays. HIV-1 antibodies were found 1985/86 in 8.5%, 1986/87 in 14.5%, 1988 in 27.7% and 1989 in 29.7% of IVDUs. The introduction of methadone maintenance in 1987 attracted many IVDUs and increased the number of patients at the out-patient clinic. In 1989 more than 50% of the IVDUs are on methadone maintenance and already 84.5% of the HIV-1 infected drug users are treated with methadone.