We reported a case of a fatal graft-versus-host disease (GVHD) which developed in a 65-year-old, male patient which was considered to have been induced by irradiated fresh blood donated by his son after a coronary bypass surgery. Fresh blood was obtained from his relatives, and a 15 Gy irradiation was performed before transfusion. The diagnosis of acute GVHD was made by clinical symptoms and histological examinations of the skin and the bone marrow. He died of sepsis on the 19th post-operative day. The HLA typing of the lymphocytes, revealed that the patient had A 2, A 24, Bw 52, Bw 62, Cw 4, DR 2, and his son had A 24, Bw 52, DR 2. A 24 and Bw 52 were homogeneous making his son histocompatible with one of the patient's haplotype. This might well be attributable to the occurrence of GVHD in this case, meaning that 15 Gy irradiation was not sufficient for the prevention of this disease.