AbstractWe report a case of NADH‐CoQ reductase deficiency, which is the first case described in Japan. A 3‐year‐and‐9‐month‐old‐boy had a severe and progressive neurologic disorder characterized by psychomotor retardation, myoclonic epilepsy and spastic quadriplegia. Moreover he had a respiratory problem of uncertain etiology in the neonatal period. Venous blood lactate levels were strikingly increased, ranging from 16.8 to 47.4 mg/100 ml, with an abnormally high lactate‐pyruvate ratio (21.2±1.7). Lactate and pyruvate levels in the cerebrospinal fluid (CSF) were also elevated (48.9 mg/100 ml and 2.09 mg/100 ml respectively). Histochemical investigation of a muscle specimen showed marked atrophy of type 1 and 2A fibers. No ragged red fiber was seen. Electron microscopy revealed a mild increase of lipid vacuoles among the myofibrils. The mitochondria showed no abnormal structure and no paracrystalline inclusion body. Biochemical studies of muscle mitochondria revealed a defective activity of NADH‐CoQ reductase in the respiratory chain. We propose that cases with so‐called mitochondrial myopathy should be classified according to the fundamental biochemical defects underlying of the clinical symptoms, and that the entity of NADH‐CoQ reductase should be divided into two further subtypes, of which one is early onset type an