Human immunodeficiency virus (HIV) type 1 has emerged during the 1980s as an important transfusion transmitted agent. The impact of HIV depends on the epidemiology and characteristics of the virus, serological response to infection and efficacy of serological tests to identify infected blood units. These factors will be described in the present article. The risk to acquire HIV today, by transfusion of anti-HIV screened blood, is extremely small in Scandinavia and the Western world. The risk can be further minimized by more sensitive screening tests and an improved donor information. The epidemiology in Scandinavia of other members of the human T-lymphotrophic viruses, HTLV-1 and HIV-2, has to be further investigated.