A 72-year-old woman with multiple recurrence of gallbladder cancer was treated by intrahepatic-arterial infusion of doxorubicin using an extracorporeal system of direct hemoperfusion with venovenous bypass. During this treatment, the patient received 600 ml of fresh whole blood and 30 units of platelet concentrate from five unrelated donors. Thereafter, high fever, skin rash over the whole body, and watery diarrhea developed, followed by leukopenia progressing to a fatal sepsis. Post-transfusion graft-versus-host disease (PT-GVHD) was suspected by the clinical manifestations and postmortem pathologic findings. To establish the diagnosis of PT-GVHD, polymerase chain reaction (PCR) amplification of DNA polymorphism associated with length variation in dinucleotide or trinucleotide microsatellite repeats at the loci of D6S89, int-2 protooncogene, and human growth factor with each of the different primer sets was performed using DNA from blood drawn from the patient with clinically established PT-GVHD of a donor origin and formalin-fixed pancreas of recipient origin. Genetic analysis revealed the changes in the patient's lymphocytes from that of the patient to that of donor origin. The present finding that formalin-fixed tissues can be used as a material of patient origin may contribute to accurate diagnosis of PT-GVHD after autopsy.