Course 1 will start with cyclophosphamide given by vein over 2-3 hours every 12 hours for 6 doses over 3 days (Days 1,2,3). Mesna will be given by vein continuously for 4 days with the cyclophosphamide to protect the bladder. Doxorubicin will be given by vein over 24 hours on Day 4. Vincristine will be given by short infusion on Days 4 and 11. Dexamethasone (a steroid) will be given by mouth or by vein on Days 1-4 and 11-14. The imatinib mesylate will be given by mouth with breakfast and a large glass of water daily on Days 1-14. Medicines will be given to prevent nausea and to protect the kidneys from increased amounts of uric acid, which may be released when leukemia cells die.