Phenylketonuria (PKU) has been aptly described as the "epitome of human biochemical genetics." In so distinguishing PKU among the many metabolic disorders now known, Scriver and Clow identified several categories in which this inborn error of metabolism is singularly prominent. First and foremost, PKU represents a fusion of effort between public health and genetics. It is the major genetic disorder in which treatment can prevent the clinical expression of disease and for which routine biochemical screening of newborn infants was developed. It remains the model for such screening. Second, PKU is the prime example of the importance of understanding as completely as possible the biochemical basis of a metabolic disorder. The detailed understanding of phenylalanine metabolism that arose from studies spawned by PKU led to the recognition of "new" metabolic disorders that relate to PKU in their capacity to increase the phenylalanine level but that involve a different category of metabolism and require very different treatment. Third, PKU represents an important link between obstetrics and pediatrics. The threat to the fetus from PKU in the pregnant woman (maternal PKU) must be met by special dietary care throughout the pregnancy. This is, perhaps, only the first of other maternal inborn errors that will require similar intervention during pregnancy.