Transfusion-associated graft-versus-host disease (TA-GVHD) is a serious, often fatal complication to the transfusion of blood components. TA-GVHD is caused primarily by donor T lymphocytes reacting towards recipient MHC antigens. The diagnosis TA-GVHD should be considered when patients, within a month of receiving blood transfusion, develop sudden, unexpected high fever and erythematous rash, possibly accompanied by gastrointestinal symptoms and/or pancytopenia. Congenital (cellular) immune defect, intrauterine transfusion, bone marrow transplantation, Hodgkin's disease, and directed transfusions (especially from first degree relatives) all carry high risk of developing TA-GVHD. Since mortality exceeds 90% irrespective of any treatment, prevention is essential. Pretransfusion gamma-irradiation of blood components with a 25 Gy dose effectively prevents TA-GVHD, and it is therefore recommended that all blood components be irradiated prior to transfusion to patients belonging to defined groups-at-risk.