Abstract
Nine children with atypical phenylketonuria are described from a Phenylketonuric Clinic of 56 children. The diagnostic criteria are (i) raised serum phenylalanine level, 4-20 mg./100 ml.; (ii) phenylalanine intolerance indistinguishable from classical phenylketonuria on loading; (iii) abnormal urinary metabolites after loading. They differ from classical phenylketonuria in their greater tolerance to dietary phenylalanine and better prognosis for future intelligence. The arguments for dietary restrictions in these infants are discussed and their dietary management outlined. Their greater tolerance to phenylalanine results in an increased danger of phenylalanine deficiency. This requires closer supervision in the first months of treatment. It is speculated that dietary restrictions will be eased by pre-school age, but that females with atypical phenylketonuria will again require dietary restrictions when pregnant.