New mitochondrial DNA mutations were discovered in the cytochrome c oxidase subunit III gene in 8 independent Leber hereditary optic neuropathy probands. A mutation at nucleotide position 9438 was found in 5 probands, changed highly conserved glycine-78 to serine (G78S), and was not found in controls. A mutation at nucleotide position 9804 was found in 3 probands, changed highly conserved alanine-200 to threonine (A200T), and also was not found in controls. The 9438 mutation is readily detected by the loss of a Stu 1 restriction site and the 9804 mutation is detected by the gain of an Mae III restriction site. These mtDNA mutations may represent the first convincing examples of cytochrome c oxidase (Complex IV) mutations associated with a human disease.