This research is being done to determine if allopurinol can change the metabolism of the oral chemotherapeutic medication 6-mercaptopurine (6-MP) in children with acute lymphoblastic leukemia (ALL). 6-MP is originally started at a standard dose in children with ALL, but the dose is adjusted according to the absolute neutrophil count (ANC). Occasionally, 6-MP doses need to be increased in order to get the ANC into a specific target range. Also, increasing the 6-MP dose can lead to unwanted side effects, such as inflammation of the liver as shown by increases in laboratory values (ALT, aspartate aminotransferase (AST), bilirubin), nausea, and abdominal discomfort. Previous studies in children with inflammatory bowel disease has shown that combining allopurinol with 6-MP can decrease side effects associated with high doses of 6-MP and also increase the efficacy of 6-MP. Allopurinol is approved by the Food and Drug Administration for the treatment of tumor lysis syndrome in ALL. Through this research study, we hope to show that the combination of allopurinol and 6-MP will be safe, tolerable, and effective in children with ALL.