The possibility of an early diagnosis of tetrahydrobiopterin deficiency among hyperphenylalaninemic infants, when specific screening tests cannot be performed, was evaluated. Three tetrahydrobiopterin deficient patients, two with dihydropteridine reductase deficiency and one with dihydrobiopterin synthetase deficiency were examined together with their parents and compared with twelve phenylketonuric patients, their parents and sixteen normal subjects. The parameters considered in the hyperphenylalaninemic patients (degree of neonatal hyperphenylalaninemia, phenylalanine lowering speed in response to a restricted diet, dietary tolerance to phenylalanine, oral phenylalanine load) were found to be insufficiently or lately indicative. By contrast, heterozygosity tests (molar ratio (Phe)2/Tyr and sigma discriminant function) performed on the parents allowed a suspicion of tetrahydrobiopterin deficiency, the definite diagnosis being of course based upon specific investigations.