We studied HIV-1 infected men who have sex with men (MSM) and drug users (DU) who participate in the ACS on HIV infection and AIDS. The HIV-infected MSM were enrolled in the cohort between October 1984 and March 1986 [35]. In the first serum sample taken at entry in the cohort, 239 men tested positive for HIV antibodies (recombinant HIV-1/-2 enzyme immunoassay (Abbott, Chicago, Illinois) with an HIV-1 Western blot IgG assay (version 1.2, Diagnostic Biotechnology Ltd., Singapore) for confirmation; five of these men refused to participate further. Of the HIV-1 negative men, 131 subsequently seroconverted during active follow-up (until May 1996). For seroprevalent individuals, an imputed seroconversion date (on average 18 months before entry into the ACS) was used [36]. Seroprevalent individuals and prospective seroconverters were studied as one group [11], [16]. Most seropositive men (n = 243 [67%]) did not receive any early treatment, 70 (19%) received zidovudine monotherapy, 10 (3%) received didanosine monotherapy, and 42 (11%) received other ineffective antiretroviral therapy before AIDS-diagnosis. The mean age of participants at the time of (imputed) seroconversion was 34.5 years
