INTRODUCTION. Phenylketonuria (PKU) is an autosomal recessive metabolic disease caused by a deficiency of phenylalanine hydroxylase. The dietary therapy for the effective management of PKU, in particular the restriction of high-protein foods of animal-origin, compromises patients' intake of fat and distorts the n-3:n-6 ratio of essential fatty acids in the diet. This deficiency can contribute to neurological and visual impairment. AIM. To evaluate changes in white matter alterations, visual evoked potential (VEP) latencies and performance in executive and motor functions in a group of early and continuously treated PKU patients after supplementation with docosahexaneoic acid (DHA). PATIENTS AND METHODS. We selected 21 PKU patients with early diagnosis (age range: 9-25 years), on a Phe-restricted diet and supplemented with PKU formula. Inclusion criteria were: low erythrocyte DHA values, prolonged P100 wave latencies in VEP and/or presence of white matter hyperintensities on brain magnetic resonance imaging (MRI), and intellectual quotient > 80. All patients were treated with DHA (10 mg/kg/day) for 12 months. Assessment was conducted at baseline and after 12 months of treatment, and included biochemical parameters, brain MRI, VEP, ophthalmologic evaluation and neuropsychological tests. RESULTS AND CONCLUSION. All the patients normalized the DHA levels after supplementation. Improvement in the P100 wave latencies, and fine motor skills was significant. No significant improvement in the other explorations was evident after supplementation. Further investigations seem advisable to establish a cause-effect relationship between DHA treatment and the slight improvement observed in some neurological functions.