Seventeen children with acute lymphocytic leukemia (ALL) in remission were treated with parenteral high-dose methotrexate (HDM) pulses every eight weeks during standard 6-mercaptopurine and methotrexate (MTX) oral maintenance therapy. MTX (1,000 mg/m2) was infused over one hour followed by one hour of intravenous hydration for the purpose of achieving plasma and cerebrospinal fluid (CSF) levels greater than 10(-6) M for a period of 24 hours. Leucovorin (15 mg/m2) was administered orally six, 12, and 18 hours after completion of the HDM. Plasma and CSF concentrations of MTX were evaluated serially in the first 48 hours. During the first 24 hours, the plasma MTX level was maintained at greater than 10(-6) M. The patients receiving intrathecal MTX at a dose of 15 mg/m2 had an adequate, sustained MTX level in the CSF, but when no intrathecal MTX was administered, the CSF levels were less than 10(-6) M. For that reason, intrathecal MTX in a low dose (6 mg/m2) was injected intrathecally one hour after the HDM infusion, allowing the MTX level in CSF to approximate 10(-6) M over the 24 hours. The toxicity of this therapy was minimal. Due to the facts that the plasma and CSF MTX levels could be sustained above the desired concentrations and this regimen could be given in the outpatient clinic, this program has been incorporated into an ongoing study in an effort to prolong complete remissions.