We describe a patient with chronic myelogenous leukemia (CML) who developed extramedullary blast crisis, and was successfully treated with imatinib mesylate (STI571). A 42-year-old man had been diagnosed with chronic phase Philadelphia chromosome (Ph)-positive CML and treated with interferon-alpha. He achieved partial cytogenetic response. Two years after the diagnosis, he presented with superficial lymphadenopathy in his neck and supraclavicular regions. Lymph node biopsy disclosed the infiltration of myeloblasts. Although the patient's bone marrow was without increasing blasts at that time, cytogenetic response was no longer observed. STI571 at a dose of 600 mg/day was initiated, and led to the complete disappearance of lymphadenopathy within a month and also to major cytogenetic response in the bone marrow (90% Ph-negative metaphases). Subsequently, the patient underwent allogeneic bone marrow transplantation from an HLA-matched unrelated donor and was in complete remission without evidence of extramedullary disease 12 months after transplantation.