Tetrahydrobiopterin (BH4) therapy is the latest alternative approach in phenylalanine hydroxylase (PAH) deficiency, and is suggested for a number of hyperphenylalaninemic (HPA) patients with certain mutations. In our unit, therapeutic efficacy of BH4 was evaluated in 20 HPA patients (4 mild HPA, 9 mild phenylketonuria-PKU, 7 moderate PKU) by a single oral dose of BH4. Overall, 60% of the patients responded (45% favorably, 15% partially). All of the mild HPA patients and 55% of mild PKU patients responded to BH4 favorably and an additional 11% of mild PKU patients responded partially. Of 7 moderate PKU patients, 2 responded partially (28%). The genotypes of the patients who responded to BH4 favorably were: DelF39/-, L48S/L48S, R261Q/- (4 patients), A300S/IVS2nt5g > c, A300S/-, E390G/E390G. The genotypes of the patients who exhibited a partial response were: L48S/L48S, R261Q/ R261Q, IVS10nt546/-. We concluded that since there are too many mutations and many patients are compound heterozygote, it is difficult to predict BH4 responsiveness based solely on genotype, especially for the mutations which show inconsistent phenotypes. The best way to identify the patients who are more likely to benefit from BH4 administration is performing BH4 loading test. Long-term BH4 loading test should be performed in classical and moderate PKU patients to confirm that they are not responsive to BH4.