Studies have been conducted on adults prescribed with methadone to determine the necessary frequency of QTc monitoring but no consensus has been reached and no similar research has been conducted in the pediatric population. The objective of this retrospective study was to determine the occurrence rate of QTc interval prolongation associated with methadone use in a pediatric oncologic population. In total, 18% of patients developed QTc interval prolongation. These patients had longer baseline QTc intervals and were on more QTc interval-prolonging medications. Our data suggest that these variables may be able to risk stratify patients who require more frequent monitoring.