The history of PKU is one of science in the discovery of an inborn error of metabolism and a chemical cause of mental retardation; and also one of technology with the development of methods to prevent disease. PKU is the classic example of success in the prevention of a genetic disease. Meanwhile, the science has continued to evolve over the 60 years since the discovery of PKU, generating new understanding of its clinical and metabolic phenotypes and about phenylalanine hydroxylation. At least five known genes are involved in hydroxylation of phenylalanine, synthesis of tetrahybrobiopterin and regeneration of this cofactor. The genes have been cloned and mutations characterized for several enzymes (GTPCH, 6-PTPS, PHS/DoCH, DHPR, PAH). A new animal model (the enu mouse) is contributing to knowledge about pathogenesis of brain disease and potential new treatments. The human phenylalanine hydroxylase gene (PAH) itself harbors 99% of the mutations causing hyperphenylalaninemia, over 170 different mutations have been identified at this locus. They cause loss of function; none affecting regulation has been identified. The aggregate PKU gene frequency at 1% is polymorphic in many human populations and mutations are highly stratified by region and population reflecting a variety of mechanisms (founder effect, genetic drift, hypermutability and, perhaps, selection) for their occurrence and distribution.