A chimeric gene, AML1-MTG16, showing high homology to AML1-MTG8, was recently identified in adult leukemic patients with the abnormal karyotype t(16;21)(q24;q22). We recently saw a child patient of 11 years of age who developed acute myelogenous leukemia with the karyotype t(16;21)(q24;q22), 11 months after autologous peripheral blood stem-cell transplantation (PBSCT) for acute promyelocytic leukemia with karyotype t(15;17)(q22;q11). The reciprocal translocation was localized by fluorescence in situ hybridization (FISH) analysis, reverse transcription polymerase chain reaction (RT-PCR), and Southern blot analysis of bone marrow blood cells and peripheral blood cells. FISH analysis identified a reciprocal translocation between chromosomes 16 and 21. RT-PCR analysis identified expression of the chimeric gene AML1-MTG16. Southern blot analysis revealed a breakpoint occurring at a 1.4 kb Eco RI fragment between exons 3 and 4 of MTG16. The breakpoint is within the same region as that of secondary leukemias, which has been reported previously. This case suggests the possibility that the region of the breakpoint of MTG16 is a characteristic of secondary leukemia.