Absent phenylalanine hydroxylase activity without phenylketonuria

A male infant is described who never manifested phenylketonuria even though phenylalanine hydroxylase activity was undetectable in liver tissue. Plasma phenylalanine were elevated in the range typical of PKU patients when the baby was at breast and declined with institution of a low phenylalanine diet. Physical and psychomotor development were normal with the baby on the latter treatment. The results indicate that the absence of phenylketonuria does not rule out phenylalanine hydroxylase activity.