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Differential diagnosis in 144 cases of hyperphenylalaninemia detected through the newborn screening is discussed. In 123 infants phenylketonuria was diagnosed, so they were treated with the low phe diet. Verificatory examinations performed in diagnostically doubtful cases with the use of protein loading confirmed persistent enzymatic defect in all of them. In 21 infants with blood serum phenylalanine level below 15 mg% and lack of phe urinary metabolites, preliminary diagnosis of mild hyperphenylalaninemia was made and they were left without dietary treatment. A decrease with age in phenylalanine and tyrosine values was observed in this group. Mental development score, in the group as a whole, at age 3-7 years was normal. Two cases with relatively low IQ values have been discussed in regard to possible reason of their mental delay.