We analyzed the clinical phenotype and determined the recurrence risks to relatives of patients with T14484C Leber's hereditary optic neuropathy (LHON). LHON is a maternally inherited optic neuropathy that primarily affects adolescent males. It is usually associated with one of three mtDNA mutations: G3460A, G11778A, or T14484C. Definition of recurrence risks for the T14484C mutation previously has not been possible due to the relative scarcity of families with this mutation. We obtained blood samples from index patients and their consenting family members, all of whom were of French Canadian ancestry and screened for LHON mutations in mtDNA. Referring ophthalmologists furnished clinical summaries and patients provided pedigree data. T14484C was the most common mutation in the pedigrees analyzed and was always homoplasmic. In these pedigrees, the ratio of affected males to females was 8:1. Median age at onset for males was 19 years (95th percentile, 40.8 years; range, 6 to 48 years). Some improvement of vision was observed in 58% of patients. Recurrence risks to brothers were 28%, sisters 5%, nephews 30%, nieces 3%, male matrilineal first cousins 19%, and female matrilineal first cousins 4%. Recurrence risks to brothers and nephews were not different; however, recurrence risks to brothers and male cousins and to nephews and male cousins were significantly different. There were no differences in recurrence risks to sisters and nieces or to either group compared with their female cousins. Affected females did not have more affected children than unaffected females. The clinical characteristics of French Canadian patients with T14484C LHON were strikingly similar to those in previous reports, suggesting that recurrence risks are generalizable to other T14484C LHON populations for genetic counseling of T14484C LHON families.