In this report, a case of transfusion-associated graft versus host disease that developed following coronary arter bypass grafting and mitral annuloplasty operations, has been presented. The diagnosis of 62 year-old male patient was based on the presence of typical findings as fever, liver function disorders, skin rash and hypoplastic bone marrow findings that began ten days after the operation; with the exclusion of other pathologies (e.g. drug eruptions, viral infections, septicemia, scalded skin syndrome and toxic epidermal necrolysis) and histopathological findings of skin biopsy. There was a history of five units of blood transfusion of which one was from a close relative. The blood from the relative was thought to be responsible for the disease. With this case, we wanted to emphasize once more that transfusion of the blood of a relative must be avoided in patients who have undergone major operations such as cardiovascular surgery. The irradiation of these bloods before transfusion may be effective to prevent graft versus host disease.