The prognosis of phenylketonuria (PKU) is related to the quality of metabolic control all life-long. PKU treatment is based on a low-Phe diet, 6R-tetrahydrobiopterin (BH4) treatment for the BH4-responsive PKU patients or enzyme replacement therapy. Fluctuations in blood phenylalanine (Phe) concentrations may be an important determinant of intellectual outcome in patients with early and continuously treated phenylketonuria (PKU). The aim of this work is to study the fluctuation of Blood Phe in patients treated by BH4 from birth in comparison with patients treated by low-Phe diet. We conducted a retrospective study in a national reference center for PKU management. We compared mean phenylalanine blood concentration and its fluctuation in 10 BH4-responder patients (BH4R) and in 10 BH4 non-responder patients (BH4NR) treated from birth. The mean blood Phe concentration is similar between the two groups before 10 years of age (290 ± 135 (BH4R) vs. 329 ± 187 µmol/L,p= 0.066 (BH4NR)) while it is lower in the BH4R group after 10 years of age. (209 ± 69 vs. 579 ± 136 µmol/L,p= 0.0008). Blood Phe fluctuation is significantly lower in the BH4R group compared to the BH4NR group (70.2 ± 75.6 vs. 104.4 ± 111.6 µmol/L,p< 0.01) before 6 years of age. There are no significant differences observed on nutritional status, growth, and neuropsychological tests between the two groups. BH4 introduced in the neonatal period is associated with less blood Phe fluctuation before 6 years. Additional time and patients are required to determine if the decrease in Phe fluctuation would positively impact the long-term outcome of PKU patients.