Phenylketonuria in the General Population

Because phenylketonuria was first recognized among severely retarded persons and their families, the finding of elevated serum phenylalanine concentrations has usually been equated with severe mental retardation. With the advent of the mass screening programs for the detection of hyperphenylalanemia in clinically normal newborn infants, one of the many troublesome questions for which we had no answer was, “Are there large numbers of people in the general population who have high phenylalanine concentrations (that is, with phenylketonuria or PKU) and who are not retarded?” This became an even thornier issue when the frequency of phenylketonuria as detected by the newborn.