We report a novel 14724G>A mutation in the mitochondrial tRNA glutamic acid gene in a 4-year-old boy with myopathy and leukoencephalopathy. A muscle biopsy showed cytochrome c oxidase-negative ragged-red fibers and biochemical analysis of the respiratory chain enzymes in muscle homogenate revealed partial complex I and complex IV deficiencies. The mutation, which affects the dihydrouridine arm at a conserved site, was nearly homoplasmic in muscle and heteroplasmic in blood DNA of the proband, but it was absent in peripheral leukocytes from the asymptomatic mother, sister, and two maternal aunts, suggesting that it arose de novo. This report proposes to look for variants in the mitochondrial genome when dealing with otherwise undetermined leukodystrophies of childhood.