We report a rare case of hypereosinophilic syndrome (HES) that developed to acute myeloblastic leukemia (AML). The patient, a 34-year-old man, presented with eosinophilia of unknown origin (white blood cells 38,200/microliter with 74% eosinophils) and pericardial effusion, and was diagnosed as having HES with a normal karyotype. He received four cycles of combination chemotherapy including cyclophosphamide, cytosine arabinoside and vindesine, and thereafter remained in remission. After 12 years, he was referred to our hospital because of fever and malaise. On admission, CBC showed white blood cells 3,000/microliter with 70% myeloblasts and 3% eosinophils. The bone marrow was hypercellular with 95% blasts, which were negative for myeloperoxidase (MPO) staining. Immunophenotype analysis revealed that the cells were positive for CD13, CD19, CD34, HLA-DR and cytoplasmic MPO. CD19-positive AML was diagnosed. Cytogenetic analysis showed 46, XY, t(6;21)(q13;q22), add(7)(q11) in 19 of 20 metaphase spreads. Rearrangement of the AML1 gene at 21q22 and fusion of the BCR/ABL gene could not be detected by fluorescence in situ hybridization analysis. The patient received combination chemotherapy and achieved a complete remission. Chromosome aberrations involving 7q as well as 21q22 suggested that the initial chemotherapy for HES might have been implicated in the pathogenesis of acute leukemia in this case.