Patients on this study will take 400 mg of imatinib twice daily (morning and evening). If you have side effects, the dose may be lowered. If you are taking less than 800 mg of imatinib, you can take your dose once per day or divided in two doses. Imatinib mesylate should be taken with a large glass of water. Bottles containing the tablets will be given to the patient every 6 months. Unused supplies must be returned at the end of the study.