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Abstract: Background: Hepatorenal tyrosinemia (HT1, fumarylacetoacetate hydrolase deficiency, MIM 276700) can cause severe hepatic, renal and peripheral nerve damage. In Québec, HT1 is frequent and neonatal HT1 screening is practiced. Nitisinone (NTBC, Orfadin ®) inhibits tyrosine degradation prior to the formation of toxic metabolites like succinylacetone and has been offered to HT1 patients in Québec since 1994. Methods: We recorded the clinical course of 78 Québec HT1 patients born between 1984 and 2004. There were three groups: those who never received nitisinone (28 patients), those who were first treated after 1month of age (26 patients) and those treated before 1month (24 patients). Retrospective chart review was performed for events before 1994, when nitisinone treatment began, and prospective data collection thereafter. Findings: No hospitalizations for acute complications of HT1 occurred during 5731months of nitisinone treatment, versus 184 during 1312months without treatment (p<0.001). Liver transplantation was performed in 20 non-nitisinone-treated patients (71%) at a median age of 26months, versus 7 late-treated patients (26%, p<0.001), and no early-treated patient (p<0.001). No early-treated patient has developed detectable liver disease after more than 5years. Ten deaths occurred in non-nitisinone treated patients versus two in treated patients (p<0.01). Both of the latter deaths were from complications of transplantation unrelated to HT1. One probable nitisinone-related event occurred, transient corneal crystals with photophobia. Interpretation: Nitisinone treatment abolishes the acute complications of HT1. Some patients with established liver disease before nitisinone treatment eventually require hepatic transplantation. Patients who receive nitisinone treatment before 1month had no detectable liver disease after more than 5years. [Copyright &y& Elsevier]