Prevalence and risk factors for human immunodeficiency (HIV) virus infection were assessed in 581 heterosexual intravenous drug users (IVDUs), recruited from eight public drug assistance centres in the northeast of Italy, an area at low risk for AIDS. The overall seroprevalence for HIV antibody was rather high, 39% (95% confidence interval (CI): 35-43%). HIV prevalence had risen from 32% in 1984-1985 to 52% in 1987-1988. Thirty-three per cent of female and 41% of male IVDUs had antibodies to HIV (p = 0.07). Seropositivity rates showed a strong east-west geographical gradient (from 16% to 60%, p = 0.005): IVDUs living in the western part of the study area had a nearly sevenfold higher risk of infection (odds ratio (OR) = 6.9, 95% CI: 4.4-13.9) than those living in the eastern part. Sharing of drug injection equipment (OR = 3.6), duration of drug addiction (OR = 2.6), use of heroin in high-incidence cities (OR = 2.3), use of cocaine in addition to heroin (OR = 1.5) and history of prostitution (OR = 2.3) increased the risk of acquiring HIV infection. Over the study period, the ORs associated with area of residence decreased, whereas those associated with the use of heroin in high-incidence cities and with prostitution increased.