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Urea Cycle Disorders ( UCD ) are among the most common genetic diseases of the metabolism and ornithine transcarbamylase deficiency (OTC), an X-linked defect is the most frequent among them. It is responsible for hyperammonemia that can lead to chronic neurological illness and potentially to death in case of delayed diagnosis and treatment. With regards to the OTC deficiency there is great clinical heterogeneity with early-onset phenotypes with mostly poor prognosis and late-onset phenotypes with a better one. In the article it is reported the case of a 8 years old patient with diagnosis of OTC deficit with late-onset phenotype. The kid was brought to our hospital because of continuous vomiting and gastro- intestinal disorders, associated with irritability and lethargy later resulted into coma. Measurement of plasma ammonia concentration, followed by measurement of plasma amino acid and urine orotic acid levels allowed to diagnose the OTC deficit, lately confirmed by molecular genetic studies. The patient has been promptly treated with Sodium Phenylbutyrate, Arginine and discontinuing the protein intake. Gradually the ammonemia value decreased, and general and neurological conditions improved with resolution of the coma. To conclude, for patients presenting unexplained neurological symptoms, confusion and decreased level of consciousness, up to coma, urea cycle disorders and in particularly OTC deficiency should be considered in the differential diagnosis and an urgent ammonia level determined. In case of hyperammonemia, the treatment should be started immediately , even without a precise ethiologic diagnosis.