Patients with phenylketonuria (PKU) have an inborn error in the metabolism of the amino acid phenylalanine (Phe) and thus must follow a strictly controlled protein-restricted diet from early infancy. This protein-restricted diet is devoid of natural dietary sources of n-3 long chain polyunsaturated fatty acids (LC-PUFA), such as eggs, meat, milk or fish. Therefore, blood concentrations of n-3 LC-PUFA, especially of docosahexaenoic acid (DHA) are reduced in PKU children compared to healthy controls. DHA availability is considered important for optimal neurological function. Previous studies have shown that neural function of PKU children is improved by high dose supplementation of fish oil providing DHA, as shown by significant improvements of both visual evoked potential latencies and of fine motor skills and coordination, but no dose response relationship has been established so far.