We report for the first time severe acute pancreatitis in a child treated for phenylketonuria (PKU) discovered on neonatal screening. This 2-year-old boy was first hospitalized for bilious vomiting and moderate back pain. Laboratory values included a lipase level of 1.142 U/L, a phenylalanine level of 10mg/dL, and computed tomography revealed Balthazar grade E pancreatitis. Continuous enteral feeding was started on the 3rd day after admission. We observed clinical and biological improvement. Etiologic investigations for pancreatitis returned negative. Despite the severity of the pancreatitis, we did not observe decompensation of the metabolic disease. Specific nutritional management was necessary.