A T-to-C transition at nucleotide (nt) 9176 in the mitochondrial adenosine triphosphatase 6 (ATPase 6) gene was detected in 2 brothers with a neurological disorder resembling Leigh syndrome. The mutation was also present in the 2 other siblings and in the mother, who were asymptomatic. In the more severely affected boy (the proband), the mutation was homoplasmic in muscle, leucocytes, and fibroblasts. In leucocytes from his affected brother, 98% of mtDNA was mutant. Heteroplasmy of varying degrees was seen in leucocytes from the mother and the 2 unaffected siblings. The mutation changes a highly conserved leucine residue near the carboxyl terminus of the mitochondrial ATPase 6 subunit to proline. It could not be detected in 168 control subjects. Studies of ATP synthesis and hydrolysis in fibroblasts from the proband were normal.