In early 1984, we treated 13 consecutive patients with acute lymphoblastic leukemia (ALL) using an induction regimen of rapidly rotated combinations of prednisone, vincristine, asparaginase, teniposide (VM-26), cytosine arabinoside, and high-dose methotrexate (MTX) followed by leucovorin rescue. The intent of this clinical trial, designated Total Therapy Study XI, is to test the hypothesis that greater initial leukemia cell kill will decrease opportunities for the development of drug-resistant mutants, with resultant improvement in the length of disease-free survival. Five patients experienced life-threatening gastrointestinal toxicity within three weeks of the start of treatment. One died. Three other patients had severe abdominal pain, abdominal distention, diarrhea, and weight loss, but not gastrointestinal bleeding. In the remaining five patients, toxicity was rapidly reversible, and each child was able to complete the planned course of chemotherapy. The study was then amended to switch high-dose MTX from the induction phase to the consolidation phase, allowing at least one week for mucosal recovery. Among the next 28 patients who were treated, none showed evidence of severe gastrointestinal toxicity. Patients now receive high-dose MTX alone as consolidation therapy and are tolerating it adequately. Drug timing should be examined critically when intensified multiple-agent regimens are being devised for initial treatment of ALL.