Science addresses ignorance; medicine uses facts. The scientific approach to phenylketonuria (PKU) led to the discovery of its causes, both ultimate (allelic heterogeneity at the PAH locus) and proximate (dietary phenylalanine), the proximal phenotype (phenylalanine hydroxylase deficiency), the associated metabolic phenotype and the major distal phenotype (impaired cognitive development and neuropsychological function) for which the pathogenesis is still being investigated. By applying knowledge through newborn screening, early diagnosis and treatment, the brain disease of PKU has been greatly ameliorated. The population approach, which converted incidence into cases, revealed genetic heterogeneity in hyperphenylalaninemia involving four other loci, controlling cofactor (BH4) synthesis and recycling, and non-random geographic distribution of mutant PAH alleles of which more than 170 were known in April 1994. Various mechanisms including founder effect, genetic drift, hypermutability and selection (perhaps) explain the polymorphic aggregate frequency (approximately 0.01) and spectrum of PKU mutations in human populations.