To assess the incidence and time trends of human immunodeficiency virus (HIV) infection among intravenous drug users (IVDUs) and to evaluate the opportunities for prevention, we studied IVDUs recruited from 23 drug dependence treatment centers in Milan and Northern Italy. Participants were screened for HIV antibodies, and seronegative subjects were enrolled. A preventive intervention, based on counseling and HIV antibody testing, was done, and participants were invited to the centers for follow-up visits. We enrolled 1,532 subjects between 1 January 1987 and 31 October 1990, and we observed 901 subjects for an average of 15.9 months. Forty-one cases of HIV infection occurred, giving a seroconversion rate of 6.1% in 1987, 4.1% in 1988, 2.2% in 1989, and 1.6% in 1990. HIV prevalence decreased from 54% in 1986 to 49% in 1989. Incidence rates were higher in areas with high prevalence. During follow-up, 35 to 55% of the subjects stopped injecting heroin intravenously altogether, and those who did not stop decreased the frequency of syringe sharing. This is probably the reason for the decline in seroconversion rates, while the apparent decline in prevalence may be due to the entry of new seronegative individuals and/or to differential withdrawal of HIV-positive individuals from the IVDU population to the heterosexual (non-IVDU) population.