Résumé : Introduction: Phenylketonuria is a recessive autosomal genetic disease. It could cause severe neuropsychiatric disorders such as intellectual disability or psychosis. We describe the case of a young patient who develop an intellectual disability and an early psychosis. Clinical case: The patient has been diagnosed by the Guthrie test at 5 days of life and was followed-up from the time of diagnosis. A poor compliance to the strict food restriction, which should be with few phenylalanine, contribute to an intellectual disability discovered by the age of 6 years. Then years later, she presented her first symptoms of early psychosis (delirium, hallucinations) and got hospitalized several times. Family medical history of schizophrenia is already known. Discussion: This case illustrates the complexity of multiples diagnoses: phenylketonuria, mild intellectual disability and psychosis from a probably mixed origin. Inpatient care in the child and adolescent psychiatry unit was necessary, to observe her behavior, to complete a medical check-up and to start an appropriate medication. In this case genetic, metabolic and environmental components are mixed up. Conclusion: The controlled diet represents the keystone of the phenylketonuria treatment. By the ways, pediatric psychiatry follow-up could be recommended for patients with such metabolic disease, which can have multiple impacts on their life. Further research are needed in both psychiatric and metabolic fields.