A 17-month-old infant presented with a 2-week history of lethargy, anorexia, and an abnormal respiratory pattern on a previous 4-month history of hypotonia and gross motor delay, suggesting a clinical phenotype of Leigh syndrome. The patient experienced no epileptic seizures. Biochemical investigations were normal other than showing evidence of inappropriate secretion of antidiuretic hormone, and cerebral magnetic resonance imaging (MRI) showed symmetrical lesions in the cervical cord and lower brain stem. Initial screening investigations for a mitochondrial cytopathy were negative. Muscle histochemistry demonstrated reduced staining of cytochrome c oxidase but no ragged red fibers. Blood and muscle-derived DNA demonstrated a high level (92% and 82%, respectively) of the m.8344A>G mutation seen in patients with the mitochondrial cytopathy, ''myoclonic epilepsy with ragged red fibers on muscle biopsy.'' This infant's early presentation including inappropriate antidiuretic secretion due to presumed renal salt loss and rapid course appears to be unique to this mutation.