Ten cases of tetrahydrobiopterin (BH4) deficiency were identified in 1,337,490 newborns screened in a Chinese population in Taiwan. The high incidence of BH4 deficiency in the Taiwanese population may be explained by a founder effect, since all of the patients revealed 6-pyruvoyltetrahydropterin synthase gene mutations, and grouping N52S and P87S mutations together constituted 88.9% of the disease alleles. BH4 supplementation with restriction of high-protein foods gave control of plasma phenylalanine within normal range, and levodopa itself prevented seizure. However, the average intelligence quotient (IQ) score of these patients was only 76 +/- 14 (56-98). Statistically, the age of starting medication, including 5-hydroxytryptophan (5-HTP), was inversely correlated to IQ scores of these patients. We suggest the combination of BH4, levodopa and 5-HTP as the standard protocol to commence the treatment of BH4 deficiency as early as possible, although prenatal brain damage could have existed.