The predominance of the T14484C mutation in French Canadians with Leber hereditary optic neuropathy is due to a founder effect. By use of genealogical reconstructions of maternal lineages, a woman married in Quebec City in 1669 is identified as the shared female ancestor for 11 of 13 affected individuals, who were previously not known to be related. These individuals carry identical mitochondrial haplogroups. The current geographic distribution of French Canadian cases overlaps with that of the founder's female descendants in 1800. This is the first example of genealogical reconstruction to identify the introduction of a mitochondrial mutation by a woman in a founder population.