 MAINTENANCE: Patients receive imatinib mesylate PO QD on days 1-84, methotrexate PO once weekly QW and IT on days 1 and 43 of courses 1, 2, and 3, and mercaptopurine PO on days 1-84. Courses with imatinib mesylate and mercaptopurine repeat every 84 days for up to 104 weeks from the start of Induction IA in the absence of disease progression or unexpected toxicity. 