Idiopathic hyperammonemia is a rare complication with a high mortality rate that occurs in persons with hematologic malignancies or hematopoietic stem cell or solid organ transplant. Patients present with encephalopathy and hyperammonemia in the absence of liver disease or inborn errors of metabolism. Several etiologies have been proposed, including chemotherapeutic agents, medications, and a catabolic state with an elevated nitrogen load in the setting of acute illness. Recently, cases of hyperammonemia in adult lung transplant recipients have been attributed to infection from Ureaplasma parvum or U urealyticum. Herein, we report a 12-year-old girl with acute myeloid leukemia and neutropenic fever who developed acute encephalopathy. Laboratory testing revealed severe hyperammonemia (blood ammonia level .1609 mmol/L) with normal liver function studies. U parvum was detected in blood, urine, and respiratory specimens by polymerase chain reaction testing. After antibiotic therapy directed against U parvum, blood ammonia levels normalized, the infection was eradicated, and the patient recovered. We propose that clinicians should test for invasive infection from Ureaplasma species in immunocompromised children with unexplained hyperammonemia.