We identified the T8993G mitochondrial mutation in a female infant who died of Leigh syndrome. The proportion of mutant mitochondrial DNA increased to near homoplasmy in three generations of the pedigree. A similarly high proportion of mutant mitochondrial DNA was found in the chorionic villi and in fetal tissues from a pregnancy interrupted because of the risk of Leigh syndrome. This study supports the concept that prenatal diagnosis can be used for Leigh syndrome with the T8993G mitochondrial DNA mutation.