Phenylketonuria, an inborn error of phenylalanine metabolism, occurs with a frequency of about 1 in 10,000 births and is treated with a strict dietary regimen. Recently, some patients with PKU have been found to show increased tolerance towards phenylalanine intake while receiving tetrahydrobiopterin (BH(4)) supplementation. We have treated two infants with BH(4)-responsive PKU with BH(4) for more than 2 years. No additional dietary control was required to maintain blood phenylalanine concentrations in the desired range. Both children have shown normal development. Generally, our results suggest that BH(4) treatment might be an option for some patients with mild PKU, as it frees them from dietary restrictions and thus improves their quality of life.