SummaryThe development of a practical screening procedure for phenylketonuria and the improvement in methods of chemical analysis have led to a realization that Folling’s (1934) disease of phenylketonuria is not a single entity.In this commentary, the current view on some aspects of phenylketonuria will be reviewed and the problems illustrated by experience gained in the Phenylketonuria Clinic at the Royal Alexandra Hospital for Children in Sydney, at present attended by fifty-six children. From April 1964, fifty-six infants and children were referred because of a positive screening test. Six were no longer abnormal at the time of retesting, forty-two had classical phenylketonuria and eight showed an atypical pattern. Subsequent family studies revealed five other classical phenylketonuric children, all of whom were retarded, and one other atypical phenylketonuric boy.