PURPOSE: To describe a patient with hereditary tyrosinemia type I (HHT-I) treated with 2-(2-nitro-4-trifluoromethylbenzoyl)-1,3-cyclohexanedione (NTBC) who developed corneal opacities. METHODS: A 14-month-old patient was diagnosed with HHT-I and began treatment with NTBC. Her serial ocular examinations were normal until age 4 years, when she developed ocular discomfort and was found to have bilateral, linear, branching subepithelial corneal opacities. RESULTS: Over the next 3 years, the extent of the opacities fluctuated, and increased opacities correlated with periods of poor compliance with a restricted protein diet. Serum tyrosine levels remained elevated at 238 to 602 umol/L (normal 26 to 83) throughout the duration of NTBC treatment. CONCLUSION: Corneal opacities are a potential consequence of NTBC treatment for HHT-I. The lesions probably result from elevated serum and ocular tyrosine levels due to inhibition of the tyrosine catabolic pathway and poor dietary compliance.