MAINTENANCE THERAPY: Within 12 weeks after beginning last course of consolidation therapy, patients receive mercaptopurine PO daily, methotrexate PO or IV over 6 hours once weekly for 2.5 years, vincristine sulfate IV on day 1 every 3 months, prednisone PO on days 1-5 every 3 months, and methotrexate IT on day 1 every 3 months. Treatment continues for up to 2.5 years in the absence of disease progression or unacceptable toxicity.