The purpose of this study is to evaluate the safety and effectiveness of imatinib for improving blood counts in patients with chronic myelomonocytic leukemia (CMML) and atypical chronic myelogenous leukemia (CML). Although a number of agents have been used to treat these diseases, most patients do not respond to treatment. Imatinib has been shown in clinical trials to induce high rates of responses in patients with chronic phase CML. Imatinib has also been shown to be effective in inducing responses in a subset of patients with CMML and atypical CML and is also effective in a subset of patients with idiopathic hypereosinophilic syndrome (HES), another myeloproliferative disorder. Because patients with several different myeloproliferative diseases have been shown to experience dramatic responses to imatinib, we would like to determine what proportion of patients with atypical myeloproliferative diseases (CMML and atypical CML) will respond to this agent. Prior to enrollment, a thorough clinical evaluation will be performed. A baseline bone marrow will be obtained to exclude acute leukemia or lymphoma and to assess the degree and nature of the myeloproliferation. In order to minimize bone marrow suppression, other myelosuppressive drugs will be tapered and discontinued during the first week of therapy with imatinib. Complete blood counts will be performed weekly for the first month and every other week thereafter. Clinical assessments will be performed every three months to assess for continued response.
* INCLUSION CRITERIA: All subjects must be greater than or equal to 18 years of age. All subjects must meet the established diagnostic criteria for CMML or atypical CML. The diagnostic criteria for CMML include: * persistent peripheral blood monocytosis (greater than 1000/mm(3)), * no Philadelphia chromosome or BCR/ABL fusion gene, * fewer than 20% blasts in the blood and bone marrow, and * dysplasia in one or more myeloid lineages. If dysplasia is absent the diagnosis of CMML can still be made if the other requirements are met and a cytogenetic abnormality is present in the marrow cells or if monocytosis has been persistent for at least 3 months and all other causes of monocytosis have been excluded. OR The diagnostic criteria for atypical CML include: * peripheral blood leukocytosis comprised of increased mature and immature neutrophils, * prominent dysgranulopoiesis, * no Philadelphia chromosome or BCR/ABL fusion gene, * neutrophil precursors greater than or equal to 10% of white blood cells, * basophils less than 2% of white blood cells, * monocytes less than 10% of white blood cells, * hypercellular bone marrow with granulocytic proliferation and dysplasia, and fewer than 20% blasts in the blood and bone marrow. * Serum creatinine less than 2mg/dl * ECOG performance status less than 3 * Life expectancy greater than 12 weeks * All subjects (men and women) must agree to practice abstinence or effective contraception during administration of imatinib. * Patients must be able to comprehend the investigational nature of the research and be willing to sign an informed consent. EXCLUSION CRITERIA: Pregnancy or lactation. HIV positivity or other known immunodeficiency. Absolute neutrophil count less than 1000/mm(3) or platelet count less than 10,000/mm(3) or less than 50,000/m(3) with clinical evidence of bleeding. Infection not adequately responding to appropriate therapy History of non-hematologic malignancy treated with chemotherapy in past 5 years. A moribund status or concurrent hepatic, renal, cardiac, metabolic disease of such severity that death within 12 weeks from initiation of therapy is likely. Treatment with investigational agent (other than hematopoietic growth factors) within 4 weeks of study entry. Psychiatric, affective, or other disorder that may compromise the ability to give informed consent or to cooperate in a research study. Elevated transaminases (greater than 5 times the upper limit of normal) or elevated bilirubin (greater than 3 times the upper limit of normal). Recent exposure to chickenpox or recent history of Herpes zoster (shingles) reactivation. Imatinib may put patients at increased risk of severe disease. Left ventricular ejection fraction less than 45%.