We distributed a questionnaire to highlight the effects of post transfusion graft-versus-host disease (PT-GVHD) and to elucidate the risk factors that would predispose people to the disease. The questionnaire described the pathogenesis and the clinical profiles of PT-GVHD and was distributed to doctors in Japan whose hospital conducted more than 1000 units of blood transfusions a year. Doctors were asked about their awareness and experience of PT-GVHD cases. Those who had seen cases of PT-GVHD were then asked to detail clinical course and laboratory data. Of the 14,083 doctors who replied to the first question, 47.4% did not realise that PT-GVHD could occur in immunocompetent hosts. From 304 cases where detailed information was supplied, 171 were considered clinically compatible with PT-GVHD. From these it seems that the risk factors linked to PT-GVHD are cardiovascular surgery, cancer, fresh and consanguineous blood transfusion, and being male. Patients with any of these factors should have some prophylactic therapy such as irradiation of blood before transfusion.