Patients with overt CNS leukemia (more or equal to 5 leukocytes per l microliter of CSF and the presence of leukemic blast cells on CSF cytospin) will receive weekly IT MHA therapy beginning 1 week after the initial dose of IT cytarabine and continuing until the CSF is free of blast cells (minimum number of doses, 4). These patients will then receive 4 additional doses of intrathecal therapy with methotrexate, hydrocortisone, and cytarabine (IT MHA) (minimum total number of doses, 8) at approximately 1-month intervals (generally given with each subsequent course of chemotherapy).