We treated two chronic phase chronic myelogenous leukemia patients with imatinib mesylate. Hematological complete remission and significant regression of bone marrow fibrosis were observed in both patients. The large amount of TGF-beta produced by increased bone marrow megakaryocytes might have caused the myelofibrosis, and it was revealed that imatinib mesylate brought about regression of the myelofibrosis by reducing the number of megakaryocytes in both patients.