Leigh disease can be caused by defects of both nuclear and mitochondrially encoded genes. One mitochondrial DNA mutation, G14459A, has been associated with both respiratory chain complex I deficiency and Leber's hereditary optic neuropathy, with or without dystonia. Here, we report the occurrence of this mutation in 3 complex I-deficient patients from 2 separate pedigrees who presented with Leigh disease, with no evidence or family history of Leber's hereditary optic neuropathy or dystonia.