Conclusion: In our case, the clinical manifestation was typical HT1. The Orfadin treatment and protein restrict diet with free tyrosine formula have improved considerably the prognosis for our HT1 patient. Objectives: Implementation of NTBC into treatment of hypertyrosinemia type I (HT I) greatly improved survival by prevention of acute liver failure and hepatocellular carcinoma. However, there are first reports of cognitive impairment in patients with elevated plasma tyrosine concentrations. Methods: Neurocognitive development in 9 patients with HT I under longterm NTBC therapy was assessed using standardized psychometric test batteries.