GROUPS 2 and 4: Patients receive compound 506U78 every 21 days for a maximum of 2 years, in the absence of disease progression. After 3 courses a patient may be given CNS prophylaxis with triple intrathecal therapy (TIT), consisting of methotrexate, cytarabine and hydrocortisone after consultation with study coordinator. TIT should be given every 12 weeks.