A patient with atypical phenylketonuria and normal liver dihydropteridine reductase and phenylalanine-4-hydroxylase activities excreted neopterin but not biopterin or dihydrobiopterin in urine. The oral administration of L-sepiapterin (1 mg/kg body weight) lowered serum-henylalanine from 17.1 to 1.1 mg/dl within 6 h. Comparable responses were observed after oral administration of L-erythro-7, 8-dihydrobiopterin or L-erythro-5, 6, 7, 8-tetrahydrobiopterin (each given in a dose of 2.5 mg/kg body weight). The results indicate a 7, 8-dihydrobiopterin synthetase deficiency in the patient.