In Thailand, the prevalence of HIV infection in the general population and in donor blood (DB) has sharply increased since 1987. The HIV seropositive rate in DB was increased from 0.0065 per cent in 1987 to 0.95 per cent in 1993 (150 times in 6 years). Heterosexual transmission is the major route of spreading. Therefore, HIV seronegative blood (SNB) poses significant hazard to the recipients because of the risk of viraemia during the window period of early HIV infection. In Thailand HIV Ab screening in all units of blood was started in 1987 and was compulsory nationwide in early 1989. Donor self exclusion (DSE) has been implemented since 1990. It is not fully effective in the prevention of transfusion associated AIDS (TAA) because of many limiting factors. However, DSE should be promoted to practice in every blood bank particularly those that can not do HIV Ag screening. During 1990-1992, there were 30 reported cases of TAA by SNB. The study of risk figure (HIV Ag positive-neutralization with HIV Ab negative) in DB was 1:3,400 and 1:10,000 in two reports in 1991. Under all these circumstances, the national AIDS committee has documented the policy to do HIV Ag screening in every unit of blood from August 1991 and allocated a 10 million baht budget (year 1992) for Ag testing. Several hospitals and NBC reported the risk figures which varied from 1:3,400 to 1:25,000. A certain amount of blood is processed to 2-4 blood components given to 2-4 patients which will increase the number of TAA by SNB.(ABSTRACT TRUNCATED AT 250 WORDS)