In many countries, numerous steps are taken to minimize the risk of infection from transfused blood products. Typically, blood banking organisations will screen for an array of infectious pathogens as part of their quality control protocol. While transmission of these tested agents via transfusion has become exceedingly rare, the risk of transfusion-transmitted infections for which testing is not currently performed continues to be a concern. Among these untested infectious agents is Epstein-Barr virus (EBV, also known as human herpesvirus-4). Most notably, infection with this virus in transplant recipients can give rise to a malignant disorder called post-transplant lymphoproliferative disease (PTLD), a life-threatening complication which is due to the uncontrolled expansion of EBV-infected cells. It is also associated with other complications such as hepatitis, hemophagocytic syndrome, etc. in transplant population. It is recognised that EBV infection can occurred in transfused immune suppressed graft recipients but the origin of the viral infection is still a matter of debate. It is a known fact that the EBV already present in the recipient's blood can undergo reactivation due to immune suppression. However, because it is known to occur more frequently in patients who are EBV-seronegative at the time of transplant, it is also accepted that primary infection contracted via an infected graft can be a source of virus. The question we are seeking to answer is whether immune suppressed graft recipients can acquire primary EBV infection via transfusion of blood products. EBV is present in the blood of most adults and cases of EBV transfusion-related infection have been reported. Transplant populations are generally transfused with very large volumes of blood products and our recent pilot study supports the possibility that transfusion-related EBV infection can be transmitted to pediatric hematopoietic stem cell (HSCT) recipients (Trottier et al, 2012). The aim of this study is to analyse the risk of EBV transmission through blood product transfusion in pediatric allogeneic HSCT patients.