Parenteral nutrition (PN) is now an important facet of the management of the extremely premature infant. However, its effects on those with inherited metabolic disease have not been well documented. We report an infant with classical phenylketonuria (PKU) who had unusually high serum phenylalanine at 12 days of life (5200 μmol;>3.2 SD above our mean for PKU at 10–15 days of age) while on parenteral nutrition, despite a relatively high tolerance for phenylalanine on oral feeds at 3–4 months of life (97–128 mg/ kg/day; normal for PKU:<90 mg/kg/day). Identification of PKU was somewhat delayed in this child because of failure to recognize that parenteral nutrition provides a phenylalanine load equivalent to or greater than the routine oral formula feeding. Despite the high levels of phenylalanine in the first 2 wk of life, mental and motor development are normal at 16 months of age. This case, the first such reported, suggests the parenteral nutrition in the premature PKU infant is relatively safe, but draws attention to the possible need for phenylalanine‐free amino acid infusates for those who require long‐term treatment.(Journal of Parenteral and Enteral Nutrition8:42–44, 1984)