In vivo phosphorus magnetic resonance spectroscopy (P-31-MRS) showed defective brain and muscle energy metabolism in three affected siblings in a family with Leber's hereditary optic neuropathy (LHON) with the 11778 mtDNA mutation. We studied 14 nonaffected members of the same pedigree by P-31-MRS and molecular genetics. Nine of 14 individuals studied had the 11778 mtDNA mutation, with various degrees of heteroplasmy. A decreased brain energy reserve, as shown by low phosphocreatine content and phosphorylation potential and high [ADP], was present in eight of these nine subjects with the 11778 mutation. A low rate of postexercise phosphocreatine recovery in muscle was present in six of the nine mutated individuals, Normal MRS findings in the brain of one and the muscle of three carriers were accompanied by a low percentage of mutated mtDNA All subjects without mutation had normal brain and muscle MRS. (31)p-MRS disclosed defective bioenergetics in the brain or muscle or both of all asymptomatic carriers studied from our pedigree.