Up to 10% of newborn children with a positive Guthrie test have non-phenylketonuria hyperphenylalaninaemia, i.e., mild elevation of serum phenylalanine that does not require dietary treatment. Depending on the relative frequencies of different phenylalanine hydroxylase mutations in a particular population, non-PKU HPA is usually caused by the combined effect of a mild HPA mutation and a severe PKU mutation. Presented here is a comprehensive analysis of non-PKU HPA in Northern Ireland. Of particular interest is one prevalent HPA mutation (T380M), which is present in over 70% of non-PKU HPA patients in Northern Ireland. Screening for this mutation is easy and inexpensive and can help confirm the diagnosis of non-PKU hyperphenylalaninaemia in the majority of cases at a very early stage. This may be clinically useful and reassuring for the parents. Other mutations described are V245A, L194P, and E390G.