We report a patient with BH(4)-sensitive phenylketonuria. In neonatal screening, phenylalanine levels above 10 mg/dl were detected. In the tetrahydrobiopterin- (BH(4)) loading test, phenylalanine concentrations in serum fell significantly. Dihydropteridine reductase activity in blood, pterines, and neurotransmitters in cerebrospinal fluid, as well as pterines in urine were all normal. Mutation analysis revealed compound-heterozygosity for the mutations R408W and K320N. Under BH(4)-supplementation without a specific phenylalanine-reduced diet, phenylalanine-concentrations are in the therapeutic range and our patient developed normally.