Patients on this study will take 400 mg of imatinib daily (morning or evening). If you have side effects, the dose may be lowered. If the response is not good, the dose of imatinib mesylate will be increased to 800 mg daily (400 mg in the morning and 400 mg in the evening) or may be decreased to 300 mg daily based on how the drug is tolerated. 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.