A 17-year-old patient had a progressive hypokinetic-rigid syndrome and several other signs and symptoms that indicated central nervous system involvement. Biochemical studies revealed a reduced form of nicotinamide-adenine dinucleotide dehydrogenase deficiency in skeletal muscle. Clinical signs and symptoms, and their association with an established defect of energy metabolism, led us to classify this disorder as a mitochondrial encephalomyopathy of Leigh's type.