Classical phenylketonuria (PKU) and mild hyperphenylalaninaemia (MHP) are two ends of the broad diagnostic spectrum in phenylalanine hydroxylase (PAH) deficiency. We have analysed a family in which classical PKU, MHP and a normal phenotype occurred in family members with different mutations. Sequence analysis revealed three mutations segregating in the family. The individual with classical PKU had two previously reported deleterious mutations. A third novel mutation was identified in the other two individuals. This report demonstrates that when discordant phenotypes occur in a family, without protein loading or phenylalanine tolerance test, complete analysis of the PAH gene may be performed in order to support the diagnosis and assist in accurate genetic counselling and patient management.