Cytochrome c oxidase deficiency is the most common biochemical defect associated with Leigh's syndrome. The genetic defect responsible for this deficiency has not been identified in any patient with Leigh's syndrome. Given that this disorder appears to be inherited as an autosomal recessive trait, this would suggest prima facie that one of the nuclear DNA-encoded cytochrome c oxidase subunits is affected. We report the first detailed sequence analysis of all 10 cytochrome c oxidase nuclear complementary DNAs and the cytochrome c oxidase mitochondrial genes in a Leigh's syndrome patient with cytochrome c oxidase deficiency. No pathological mutations were identified in any of the cytochrome c oxidase structural genes.