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.