Transfusion-associated graft-versus-host disease (TA-GvHD) is a rare complication of blood transfusion that has a fatal outcome in most patients. It is caused by the transfusion of viable T cells present in blood products that are not rejected by the transfusion recipient, either because of recipient immunodeficiency or because of a common HLA haplotype between the blood donor and recipient. Because effective treatment is not available, risk identification and prevention are of central importance. Among the potential risk factors that have been discussed to date, a definite hazard for developing TA-GvHD has been recognized for HLA-matched transfusions or transfusions from blood relatives, intrauterine and exchange transfusions, patients with congenital immunodeficiency syndromes, bone marrow transplantation, stem cell transplantation, or lymphomas. Patients at possible TA-GvHD risk who will require further evaluation include patients with hematologic malignancies, solid tumors, or solid organ transplantation. Although postulated, an increased risk for term or preterm newborns and patients with HIV/AIDS has not thus far been demonstrated.