BACKGROUND We describe the results of donor blood screening in Denmark for anti-HIV-1/2 in the period 1990-1999 and for anti-HCV, HBsAg, and anti-HTLV-I/II in the period 1994-1999. METHODS Danish blood banks report the results of donor blood screening to the State Serum Institute on a monthly basis. This study concerns the data reported in the above two period. Reports from the years given above are worked out in the present study. RESULTS The frequencies of infection among candidate donors are for HIV: 4.8 per 100,000; hepatitis C: 52.7 per 100,000; and hepatitis B: 60 per 100. The yearly incidences of new infections among the 260,000 regular donors in Denmark are for HIV: 0.42 per 100,000 regular donors; hepatitis C: 0.64 per 100,000 regular donors; and hepatitis B: 0.70 per 100,000 regular donors. All donations in Denmark were tested for anti-HTLV-I/II in the period 1 April 1994 to 1 October 1997, after which only candidate donors were tested. The frequency of HTLV-I infection in previously untested donors was 1 in 60,000 and that of HTLV-II infection 1 in 150,000. The frequency of HTLV-I infection among candidate donors was 1 in 50,000 and that of HTLV-II infection 1 in 100,000. No seroconverters have been found.