The presence of asymptomatic shedding of certain infectious agents in the bloodstream poses a risk of transmission of these agents during blood transfusions. While this risk is currently well controlled for certain infectious agents (HIV, HCV, HBV), it is not, however, documented or quantified for other pathogens responsible for infections that are either undetected or undetectable by serology at the time of donation. This risk is generally low in endemic settings, but it increases over time and across regions when clusters of cases or epidemics occur. Preventive measures can then be implemented (suspension of collection, quarantine of donations, etc.). Since these measures can have significant repercussions, particularly by limiting the supply of labile blood products to healthcare facilities, it is important that they be tailored to the risk of transmission via transfusion. Quantitative estimates of the risk of contamination in a blood donation can thus help guide these decisions. It is in this context that the InVS initiated, in early 2005, as part of a working group involving the French Health Products Safety Agency (Afssaps), the French Blood Establishment (EFS), the National Institute of Blood Transfusion (INTS), and the Institute for Public Health Surveillance (InVS), this project aimed at providing a priori quantitative estimates of the risk of blood donation contamination by infectious agents for various scenarios, in terms of incidence and spatio-temporal distribution. (R.A.)