Tetrasomy 8 is a rare chromosomal abnormality in acute leukemia, and it has recently been considered as a poor prognostic factor. A 20-year-old woman was admitted because of purpura on the upper and lower limbs in February 2002. On admission, her leukocyte count was 6.5 x 10(9)/l with 66% of blasts, the hemoglobin level was 11.2 g/dl, and the platelet count was 101 x 10(9)/l. The bone marrow aspirate contained 85.6% of peroxidase-negative, alpha-naphthyl-butyrate esterase-positive, and CD4+ CD56+ blast cells. Karyotypic analysis of the bone marrow cells showed 48, XY, + 8, + 8[17]/47, XY, +8[3]. The patient was diagnosed as having AML (M5a), and treatment with daunorubicin (70 mg x 5 days) and cytosine arabinoside (150 mg x 7 days) resulted in a complete remission. She relapsed four months later, however, with an extramedullary tumor in T12. Remission could not be achieved, and the patient underwent allogeneic peripheral blood stem cell transplantation from her HLA-identical mother. Her clinical course was almost uneventful except for a phlegmon in the right leg, but on day 49 a relapse occurred, and she died of acute renal failure on day 73. This case strongly illustrates the characteristic of tetrasomy 8 as a poor prognostic factor in acute leukemia.