Maternal hyperphenylalaninemia constitutes a potential hazard to the fetus for whom the risks of postnatal mental retardation, microcephaly, and congenital malformations are elevated. Preconception and intragestational dietary treatment can apparently improve the outcome of such pregnancies. In the absence of predictive mechanisms for pregnancies at risk and preventive measures involving reproductive counseling and treatment, there could be a rebound in the population frequency of mental retardation related to disorders of phenylalanine metabolism in subsequent generations. We describe a program serving a population of six million that includes screening, diagnosis, treatment, and counseling of the hyperphenylalaninemias. The program has recently added a simple dedicated register for males and females with hyperphenylalaninemia to supplement traditional methods for continuous surveillance of probands. We registered 153 patients: 43 females and 56 males with phenylketonuria, 23 females and 31 males with benign hyperphenylalaninemia, of which 22, 7, 27 and 5, respectively, had reached their 12th birthday in an 1981. Regional centers in the program provided counseling about the consequences of maternal hyperphenylalaninemia and the options to prevent them. No family has rejected the principle or fact of the Register and its goals.