MAINTENANCE: Following DELAYED INTENSIFICATION PART II, patients receive mercaptopurine PO or NG on days 1-168, methotrexate IT on day 1 and 92 and PO once weekly on days 8-91 and 98-168, hydrocortisone sodium succinate IT on day 1, 57, and 99, and cytarabine IT on day 57. Starting on day 169, patients receive mercaptopurine PO or NG on days 1-84 and methotrexate PO once weekly. Courses repeat every 84 days for 2 years from the start of INDUCTION CHEMOTHERAPY in the absence of disease progression or unacceptable toxicity.