Phenylketonuria (PKU) leads to severe neurological disorders in childhood, shunned by the diet. The long-term prognosis after diet diversification at adolescence is uncertain. We report a case of cortical blindness in a young patient regressive 1 month after the diet was resumed.Mr M., 25 years old, had PKU detected at birth. He maintained good serum levels of Phenylalanine (Phe) (120-300 μmol/L) during childhood and got a normal intellectual development. During adolescence he diversified his diet but maintained low meat and fish intake; Phe was ~1,200 μmol/L with no symptoms. In 2009, the patient stopped the low-Phe amino acid substitutes due to weariness. On June 27, 2011, he consulted for a decrease of visual acuity progressing for 6 months. Ophthalmologic examination found that visual acuity was 2/10 in two eyes associated to a central visual field defect. The visual evoked potentials were altered. MRI showed bilateral and symmetric occipital FLAIR hyperintensities. On admission in the Nutrional Unit on June 29, 2011, blood pressure was 120/70 mmHg, there was no other neurological abnormality. Phe was at 1,512 μmol/L, and not responsive to BH4. He was then treated with a very low-Phe diet with an amino acid substitute, and he obtained Phe between 120 and 300 μmol/L. Visual acuity was suddenly restored on August 1, 2011, with a dramatic attenuation of the MRI hyperintensities.Our observation shows that the withdrawal of the diet and substitutes exposes to serious neurological complications in adults that may reverse with a fast nutritional support.