Tyrosinemia I (fumarylacetoacetate hydrolase deficiency) is an autosomal recessive inborn error of tyrosine metabolism that produces liver failure in infancy or a more chronic course of liver disease with cirrhosis, often complicated by hepatocellular carcinoma in childhood or early adolescence. We studied a 37-year-old woman with tyrosinemia I whose severe liver disease in infancy and rickets during childhood were resolved with dietary therapy. From 14 years of age, she resumed unrestricted diet with the continued presence of the biochemical features of tyrosinemia, yet maintained normal liver function. In adult years, she accumulated only a small amount of succinylacetone. Despite this evolution to a mild biochemical and clinical phenotype, she eventually developed hepatocellular carcinoma. Her fumarylacetoacetate hydrolase genotype consists of a splice mutation, IVS6-1G>T, and a novel missense mutation, p.Q279R. Studies of resected liver revealed the absence of hydrolytic activity and immunological expression of fumarylacetoacetate hydrolase in tumour. In the non-tumoral areas, however, 53% of normal hydrolytic activity and immunologically present fumarylacetoacetate hydrolase were found. This case demonstrates the high risk of liver cancer in tyrosinemia I even in a seemingly favorable biological environment. In this study of tyrosinemia I, Case 2 with negative succinylacetone accumulation and the recovery of acute liver failure was compared with Case 1. Diet restriction and NTBC treatment are crucial to prevent hepatocellular carcinoma until liver transplant can take place and cure the condition. Further studies are needed to examine cases where liver cancer did not result despite clinical symptoms/signs of tyrosinemia type I.