Stored blood samples from 164 intravenous drug abusers who attended a Scottish general practice were tested for HTLV-III/LAV (human T cell lymphotropic virus type III/lymphadenopathy associated virus) infection. Of those tested, 83 (51%) were seropositive, which is well above the prevalence reported elsewhere in Britain and Europe and approaches that observed in New York City. The timing of taking samples of negative sera and continued drug use suggest that as many as 85% of this population might now be infected. The infection became epidemic in late 1983 and early 1984, thereafter becoming endemic. The practice of sharing needles and syringes correlated with seropositivity, which, combined with the almost exclusive intravenous use of heroin and other behavioural patterns, may explain the high prevalence of HTLV-III/LAV infection in the area. Rapid and aggressive intervention is needed to control the spread of infection.