Sapropterin enhances phenylalanine hydroxylase activity, thus lowering blood phenylalanine (Phe) concentration while increasing protein tolerance in sapropterin-responsive patients. Initiation of sapropterin treatment in responsive patients as early as possible, especially during the time when brain development is fastest, allows intake of more natural protein as well as micro- and macronutrients. Initiation of sapropterin treatment in the newborn period can make exclusive breastfeeding possible. Reports on the efficacy and safety of sapropterin in phenylketonuria (PKU) children under age four are limited in the literature. The purpose of this study is to evaluate the efficacy and safety of sapropterin treatment in infants and children with hyperphenylalaninemia (HPA) and to assess whether genotype analyses are of help in the prediction of responsiveness in these children. Clinical features as well as dietary characteristics were examined in 44 patients undergoing sapropterin treatment. Molecular genetic analysis was performed in 28 of these patients. Phe tolerance increased a median of 2.26-fold (0.88-4.23), from a median of 47.5 mg/kg/day to a median of 114 mg/kg/day (p<0.001). Phe levels could not be kept within normal limits in 5 patients, and thus treatment was stopped due to unsatisfactory metabolic control. In 9 patients, sapropterin treatment was started prior to the initiation of a Phe-restricted diet. Sapropterin treatment was found to be safe and efficacious in patients under age four. Although the BH4 loading test and molecular genetic analysis proved to be useful in detecting responsive patients, these analyses did not enable us to make predictions as to long-term responsiveness.