Transfusion-associated graft-versus-host disease is a rare but usually fatal complication of transfusion of cellular blood components, caused by multiorgan engraftment and proliferation of donor T lymphocytes. The classical features of skin rash, diarrhoea and hepatitis, along with striking bone-marrow failure, are seen 1-2 weeks after transfusion. Although early reports described the condition only in immunosuppressed individuals, sharing of an HLA haplotype between donor and an immunocompetent recipient can also result in transfusion-associated graft-versus-host disease. The condition is entirely preventable by gamma irradiation of cellular blood components to 25 Gy, although this results in some reduction of red-cell viability and increased loss of red-cell potassium. The major indications for irradiated blood components include bone marrow/stem cell auto- or allografting, Hodgkin's disease, intrauterine transfusions, and transfusions from relatives or HLA-selected platelet donors.