A multicentre cohort study was carried out to estimate the incidence of AIDS and HIV-related conditions in newly infected intravenous drug users (IVDU). The enrollment criteria included the identification of the seroconversion time. Two hundred and five subjects entered the study, and were followed for a mean of 26 months. Twelve subjects developed clinical AIDS over a 4-year period. The actuarial incidence of AIDS estimated by Kaplan-Meier survival technique was 17.8% by 4 years since seroconversion. The risk of developing AIDS increased significantly after 24 months from seroconversion. Relatively small figures accounted for the lack of statistical association between the risk factors investigated and the disease status.The natural course of HIV infection in 205 intravenous drug users (IVDU) presenting at 11 centers in Italy was followed since 1983. Criteria for entering the study were spontaneous attendance at the clinic from January 1983-April 1988; history of IVDU; dated negative HIV test and a positive test not over 18 months later. 13 of the 205 subjects developed AIDS by 30 April 1988, a total of 447.25 person-years, for an overall incidence of AIDS of 2.9% person-years. The clinical presentation was opportunistic infections in 7, wasting syndrome in 5, and encephalopathy in 1. No Kaposi's sarcoma was seen. The incidence of AIDS was 1.5% in the 1st year; 2.9% after 2 years; 9.9% after 3 years and 17.9% after 42-48 months. The risk of AIDS increased significantly 24 months after seroconversion. 11 others developed AIDS-related complex (ARC) in the 4-year period, an incidence of 3.4% person-years. The minor opportunistic infections seen were zona (4 cases), oral candidiasis (2), and hairy leukoplakia (1). 2 subjects had myelopathy. 87 others developed persistent generalized lymphadenopathy (PGL). 45.8% of the group remained asymptomatic. The subjects mean age was 26, and the sex ratio was 4:1 overall, but 12:1 among the AIDS group. The natural history of AIDS in this series closely resembled other reports on male homosexuals.