Mutation at the phenylalanine hydroxylase (PAH) locus is a cause of hyperphenylalaninaemia. Genotype-phenotype correlation relative to the predicted PAH activity may differ at the metabolite level and at the IQ level in untreated phenylketonuria. Discordant metabolic phenotypes have been noted in siblings; influences on transport and metabolism of phenylalanine determining homeostasis may account for differing metabolic phenotypes. We report two siblings of different sex and identical genotype at the PAH locus who demonstrate a difference in phenylalanine disposal. A stable isotope infusion of [2H5]phenylalanine was used to measure protein turnover, phenylalanine hydroxylation and excretion of phenylalanine transamination metabolites. The siblings were observed to have identical hydroxylation rates under the experimental conditions of the study while manifesting differences in renal excretion rates of phenylalanine transamination metabolites and protein accretion.