The amino acid requirements of the parenterally fed neonate are poorly defined. Newborn infants are at risk for amino acid deficiency and toxicity, due to lack of small intestinal metabolism and metabolic immaturity. We discuss recent evidence that identifies inadequacies of commercial amino acid solutions with respect to the balance and quantity of aromatic amino acids, and sulphur amino acids. We present data demonstrating that impaired small intestinal metabolism (or lack of first pass metabolism) alters the whole body requirement for methionine, threonine, and arginine, and discuss the potential adverse effects of excess or inadequate parenteral amino acid intake.