A 29-year-old man was referred to our hospital with leukocytosis on March 7th, 2002. The white blood cell count was 132.9 x 10(3)/microliter with 42.0% blast cells. We diagnosed Philadelphia chromosome-positive chronic myelogenous leukemia (CML) in a blast crisis and started imatinib mesylate therapy at a dose of 800 mg/day on March 9th, 2002. The patient's peripheral blood blasts had disappeared by March 22nd, 2002, and the percentage of blasts in the bone marrow was 0.6% on May 2nd, 2002. The patient achieved a complete cytogenetic response on May 13th, 2002, and underwent allogeneic peripheral blood stem cell transplantation from his HLA-identical sibling donor on May 30th, 2002. Although adverse reactions such as grade 3/4 of hematological events (leukopenia, anemia, thrombocytopenia and neutropenia) and grade 1/2 of non-hematological events (hyperbilirubinemia, dermatitis and edema) were observed, these adverse reactions were clinically managed. This case suggested the usefulness of imatinib mesylate in the management of the CML-associated blast crisis.