In the period 1st September 1974 to 1st January 1989 2,511,000 infants (99.3% of all livebirths) were screened for PKU. Treatment was required in 158 of the 330 referred infants. The diagnosis of these 158 patients was PKU (137), hyperphenylalaninaemia (HPA) (19) and malignant HPA (2). Three cases had a false-negative screening result. The prevalence of PKU, treated HPA and malignant HPA in the Netherlands is 1:18,000, 1:132,000 en 1:1,300,000. Height adjusted for age was in 24% of the patients with PKU of Dutch parents and weight adjusted for height in 11% under the 10th centile. Ninety per cent of the patients with PKU of 5 years and older attended primary or secondary school, compared with 95% in the general population.