Beside the interpretation of inverse diurnal variation of serum phe (Güttler et al, 1969) , the question of catabolic states has been considered only very rarely in reports and guidelines on PKU management. Distinguishing such states from non-compliance may motivate compliant patients and their families to even stronger adhere to their dietetic schedule. Objectives: We report the first case of Munchausen by proxy in phenylketonuria (PKU). Case report: We report the case of a PKU boy whose metabolic follow up was difficult in the first year of life. He was referred to our centre for the management of very high Phe level (3300 μmol/L) which were non responsive to classical treatment. Despite an emergency regimen (PKU aminoacids substitutes+high caloric intake and 0 Phe intake) and a weight gain was 1.7 kg, the Phe level remained increased for 3 weeks. As we were suspicious of Munchausen by proxy, we asked to the mother to leave the hospital for a week and Phe level normalized within 3 days. Accidentally, we found in the child's bag, a bottle of a high caloric, high protein formula which could be the source of the extra protein given to the child. After the child was retired from his family, the Phe level never rise up again. Conclusion: Munchausen by proxy must be evoked when there is a discrepancy between the administered diet and the metabolic status and after elimination of all the other causes of metabolic decompensation.