A multicentre cohort study was conducted in Italy to estimate the risk of developing AIDS in 261 intravenous drug users and 89 homosexual males for whom the seroconversion period was known. Four years after HIV seroconversion, AIDS incidence, estimated by Kaplan-Meier survival technique, was 13.8% for intravenous drug users and 16.2% for homosexual males; the difference was not statistically significant. These findings suggest that four years after seroconversion the risk of developing AIDS in HIV seropositive intravenous drug users is no higher than that of subjects who acquired HIV infection through sexual contact.