This study will evaluate the safety and effectiveness of imatinib (Gleevec(Registered Trademark)) in patients with chronic myelomonocytic leukemia (CMML) and atypical chronic myelogenous leukemia (CML). These conditions cause uncontrolled growth of malignant (cancerous) cells in the bone marrow that prevents the bone marrow from functioning normally in producing blood cells. The cancer cells also can spill over into the blood and invade other organs of the body. Imatinib has been approved by the Food and Drug Administration for treating chronic myelogenous leukemia, which has characteristics similar to atypical CML and to CMML, and data from other research suggests this drug may be able to produce a remission in forms of leukemia other than CML.