Palifermin and pegaspargase are Escherichia coli-derived drug products. Hypersensitivity reactions, including anaphylaxis, are frequently reported with pegaspargase. In high-risk acute lymphoblasic leukemia (ALL), patients undergoing allogeneic hematopoietic stem cell transplant may be treated with palifermin as a supportive care measure for mucositis prophylaxis. However, no literature exists documenting the cross-reactivity between palifermin and pegaspargase. We report a case in which a child with very high-risk ALL having experienced severe anaphylaxis with pegaspargase was later successfully treated with palifermin during stem cell transplant conditioning.