Three children with hyperphenylalaninaemia and hyperphenylalaninaemic mothers are presented. At least one of the affected children was a compound heterozygote for hyperphenylalaninaemia and phenylketonuria. The families were examined by an L-phenylalanine loading test, by direct determination of phenylalanine hydroxylase and/or a loading test with hepta-deuterophenylalanine. We conclude that most of the patients with moderately elevated serum phenylalanine should have the genotype hyperphenylalaninaemia/phenylketonuria, i.e. they are compound heterozygotes.