A 1-month-old Japanese girl had profound generalized weakness, hypotonia, and severe lactic acidosis. The infant improved gradually: she held her head at 9 months, learned to walk by 15 months. At the first muscle biopsy at 11 weeks of age, the specimen was characterized by numerous ragged-red fibers and decreased enzyme activity on cytochrome c oxidase (COX) staining. Electron microscopic findings were characterized by the presence of excessive abnormal mitochondria not only in skeletal muscle fibers but also in blood vessels. Vascular abnormalities consisted of an increased number of enlarged mitochondria in endothelial and smooth muscle cells of small arteries. Biochemical analysis showed an isolated defect of COX activity, which was only 16% of the mean control level. At the second biopsy at 44 months of age, the COX activity had increased to normal in the entire specimen. On electron microscopy, the abnormal mitochondria present on the first biopsy specimen had disappeared both in muscle fibers and blood vessels; nearly all mitochondria were morphologically normal at the second biopsy. Now at 5 years of age she can run and does not show muscle weakness. We report reversibility of abnormal mitochondria with age not only in skeletal muscle fibers but also in blood vessels in a patient, who had reversible COX deficiency with a benign clinical course.