To examine the incidence and prevalence of HIV infection, we studied a large sample of intravenous drug users (IVDUs) attending a drug dependency unit in Rome over the period 1985-1989. The annual prevalence of HIV antibodies remained stable over the 5-year period. However, a seroconversion study conducted on 302 subjects consistently attending the same facility showed a continued occurrence of HIV seroconversion, although the incidence declined from 8.9 per 100 person-years in 1985-1987 to 5.3 per 100 person-years in 1987-1989. The cumulative incidence of HIV seropositivity, estimated by the Kaplan-Meier survival technique, was higher in female than in male IVDUs. The findings show that the use of both incidence and prevalence data to monitor the trend of HIV infection allows a better understanding of the current viral spread among IVDUs.