* Patients will have several visits to the Pediatric Oncology outpatient clinic. Each visit will consist of a physical examination and laboratory evaluation. Each laboratory evaluation will consist of taking approximately 10-15 milliliters of blood (or approximately three teaspoons). These clinic visits may actually coincide with clinic visits that were previously scheduled according to the patient's treatment protocol.
* At the first study visit, patients will have a physical examination and laboratory evaluation. At that visit, patients will be asked to stop taking 6-MP and methotrexate.
* At the second study visit, which is one week later, patients will again have a physical examination and laboratory evaluation. The investigators will prescribe allopurinol and restart 6-MP and methotrexate at half of the patient's previous doses.
* Clinic visits for physical examination and laboratory evaluation will be scheduled every 1-2 weeks for a total of 5 more visits. Doses of allopurinol, 6-MP, and methotrexate may be adjusted at these visits based on laboratory values or clinical symptoms.
