Two infants who had clinical and radiographic findings consistent with Leigh syndrome were found to have deficiency of complex I (reduced nicotinamide-adenine dinucleotide--coenzyme Q reductase) activity. Significant abnormalities were found on computed tomographic scans and magnetic resonance images of the brain. Lactate and pyruvate concentrations in blood and cerebrospinal fluid were elevated, and muscle biopsy specimens showed abnormal mitochondria. These data indicate that Leigh syndrome, as well as MELAS syndrome (mitochondrial encephalopathy, myopathy, lactic acidosis, and stroke-like episodes) may result from complex I deficiency.