A newborn male presented with severe respiratory insufficiency, generalized muscle weakness, and lactic acidemia. Immediately after admission, he was placed on a respirator because of respiratory arrest. He deteriorated rapidly and died 75 hours after birth. There was notable variation in fiber size and an increased number of type 2C fibers in the quadriceps femoris muscle obtained at autopsy; however, no ragged-red fibers were observed with modified Gomori trichrome staining. Markedly decreased cytochrome c oxidase activity was demonstrated in skeletal muscle by biochemical and histochemical studies, while cardiac muscle demonstrated normal cytochrome c oxidase activity. Mitochondrial myopathy should be considered in the differential diagnosis of patients with neonatal respiratory distress syndrome.