A 3-month-old female infant had profound generalized weakness, de Toni-Fanconi-Debre syndrome, and lactic acidosis. She required assisted ventilation and died at the age of 8 months. Muscle biopsy showed accumulation of mitochondria, glycogen, and lipid droplets. Histochemical reaction and immunocytochemical stain for cytochrome c oxidase showed very weak results, but both reactions were normal in intrafusal fibers of the muscle spindle. In crude extracts of the patient's muscle, cytochrome c oxidase activity was undetectable and enzyme-linked immunosorbent assay showed decreased reaction at all dilutions of antiserum. These data indicate that the amount of immunoreactive enzyme protein is markedly decreased in muscle of patients with fatal infantile cytochrome c oxidase deficiency and renal dysfunction.