A 3-year-old girl developed fever and bilateral exophthalmos. Her initial peripheral blood counts on admission were; Hb8.7 g/dl, Plt. 10.6 x 10(4)/microliters, and WBC 30,890/microliters with 31% peroxidase-positive blasts, some of which contained Auer rods. The bone marrow smears showed 22% blasts with a certain degree of maturation. She was diagnosed as having M2 type AML. Computed tomography revealed bilateral retoro-orbital tumor. The bone marrow karyotype showed no 8;21 translocation but a loss of one sex chromosome. Molecular analysis of the bone marrow cells disclosed rearrangements of the AML1 gene. Chimeric mRNA coded by the AML1-MTG8 gene was also detected by the RT-PCR method. We concluded that this patient had a masked 8;21 translocation in her leukemic cells.