This study estimates the risk of transmitting human immunodeficiency virus (HIV), human T lymphotropic virus (HTLV), hepatitis C virus (HCV) and hepatitis B virus (HBV) from blood units using a seroconversion incidence model. Data from 13 blood transfusion centers collecting about 1 million donations per year and belonging to the Retrovirus and the Viral Hepatitis study groups were analyzed during the 3 year study period (1992-1994) for HIV, HTLV, and HBV and a 2 year study period for HCV (1993-1994). Seroconversion incidence rates were calculated and multiplied by estimates of the serological window period for each agent to obtain residual risk. The risk that an infectious donation was made during the window period was estimated to be 1 in 2 millions (95% CI: 1/10(7)-1/450000) for HTLV, 1 in 588000 (1/3 300000-1/227000) for HIV, 1 in 217000 (1/714000-1/83000) for HCV and 1 in 112000 (1/333000-1/43500) for HBV. This risk was estimated for the totality of donations collected in France for HIV and HTLV. For HIV it was the same as above (1 in 588000) and for HTLV it was much lower (1 in 7 millions).