The Pediatric Oncology Group 9203 pilot protocol was designed to determine the feasibility of delivering 29 biweekly doses of polyethylene glycol (PEG)-L-asparaginase, on a backbone of intensive multiagent antimetabolite-based consolidation and maintenance in higher risk B-precursor acute lymphoblastic leukemia. Between June 1992 and August 1993, 34 patients were enrolled on this limited institution pilot. The 5-year event-free survival (±standard error) and overall survival (±standard error) were 68±8% and 76±7%, respectively. Excessive toxicities attributed to PEG-L-asparaginase and myelosuppression associated with cytosine arabinoside were encountered during consolidation resulting in early study closure and modification of therapy for those already enrolled. Ninety-two percent of methotrexate/cytosine arabinoside cycles were associated with grades 3 to 4 myelosuppression, and 24% resulted in delays in therapy of more than 7 days. Fifteen PEG-L-asparaginase related toxicities occurred in 13 patients (8 allergy, 4 pancreas, 2 central nervous system, and 1 hemorrhage). Intensification of therapy with PEG-L-asparaginase resulted in event-free survival and overall survival comparable to other studies of the same time period without the use of agents associated with long-term complications, such as anthracyclines, epipodophyllotoxins, and alkylating agents. However, excessive toxicity occurred with intensified PEG-L-asparaginase and antimetabolite based therapy delivered on this schedule.