By combining data collected through a cohort study among injecting drug users (IDUs), which started December 1985, with data on the number of notified cases of AIDS, the cumulative number of HIV-infected IDUs residing in Amsterdam was estimated. The estimated cumulative incidence of HIV-infection among IDUs in Amsterdam on 1 July 1991 was about 1050. By 1 July 1991 90 of these cases had been diagnosed with AIDS, while an estimated 150-200 cases died before AIDS had been diagnosed. The prevalence of IDUs residing in Amsterdam who were still alive and free of AIDS on 1 July 1991 was estimated to be about 750-800. Because of untimely deaths, only a limited number of these HIV-infected IDUs can be expected to be diagnosed with AIDS in the future. The prevalence on 1 July 1991 of HIV-infected IDUs being alive and free of (diagnosed) AIDS and living elsewhere in the Netherlands is estimated at about 500.