Patients with hyperphenylalaninemia appear to represent a heterogeneous group. The enzyme phenylalanine hydroxylase may be implicated, either by indirect measurement of activity (phenylalanine loading studies) or through direct enzyme assays in the patients studied. When phenylalanine tolerance tests were used, the measurement of phenylalanine levels alone did not distinguish the subject with classical phenylketonuria from the atypical subject. Rise in plasma tyrosine levels as well as the excretion of breakdown products of phenylalanine and tyrosine better distinguished the two types of subjects. A significant rise in plasma tyrosine level was demonstrable in the atypical subject in sharp contrast to the classical homozygous subject with phenylketonuria.