 INTERIM MAINTENANCE: Patients receive imatinib mesylate PO QD on days 1-63, vincristine sulfate IV over 1 minute and high dose methotrexate IV over 24 hours on days 1, 15, 29, and 43, leucovorin calcium PO or IV on days 3 and 4, mercaptopurine PO on days 1-56, and methotrexate IT on days 1 and 29 in the absence of disease progression or unexpected toxicity. 