Acetaminophen poisoning remains a leading cause of acute liver failure. Some cases may be difficult to diagnose when clinical findings are equivocal, the patient's history is misleading or incomplete, or the acetaminophen concentration is low or undetectable at arrival to care. We describe a case of a medically complicated young child whose acute liver failure had two of these features. Measurement of acetaminophen-protein adducts confirmed acetaminophen as the primary cause of her liver failure.