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We report a 23-year-old man with mitochondrial encephalomyopathy. At 21 years of age, he noted speech distubance. Since his dysarthria did not improve thereafter, he was admitted to our hospital. On admission, he showed mild gynecomastia. Neurological examination revealed mild decrease in performance IQ in WAIS-R, mild scanning speech, mild left hearing disturbance, mild to moderate muscle weakness in proximal four extremities, mild bilateral limb ataxia, and mild to moderate truncal ataxia. While, no brisk deep tendon reflex, pathological reflex, aberrant muscle tonus, sensory disturbance, retinopathy, myoclonus or autonomic disorder was found. Serum levels of lactate (23.2 mg/dl, normal<18.7) and pyruvate (1.23 mg/dl, normal<0.94) were elevated, and serum lactate levels were markedly elevated (118.1 mg/dl) after 15-minute exercise (15 Watts/minute). CSF levels of lactate (31.2 mg/dl, normal<12.5) and pyruvate (1.48 mg/dl, normal<0.75) were also elevated. Head MRI showed mild cerebral and cerebellar atrophy, but 1H-MRS showed no lactate peak. Moreover, muscle biopsy from left biceps muscle showed lots of ragged-red fibers, and he was thus diagnosed as having mitochondrial encephalomyopathy. However, nt3243 mutation of mitochondria DNA was not present. Next, we confirmed gynecomastia by mammography, and checked serum levels of estrogens. Mildly decreased estradiol (19.9 pg/ml, normal, 20-59), normal estrone (24.0 pg/ml, normal<30.0) and mildly increased estriol (6.03 pg/ml, normal<5.0) were found. While, the serum levels of cortisol, dehydroepiandrosterone-sulfate (DHEA-S), androstenedione, testosterone, luteinizing hormone (LH) and follicle stimulating hormone (FSH) were all within normal limits. Since the steroid hormone synthesis system and hypothalamus-pituitary system seem to be normal, 16alpha-hydroxylase that converts estradiol to estriol may be upregulated. While, aromatase (P-450arom) is well known to convert androgens to estrogens. In addition, 16alpha-hydroxylase and P-450arom convert DHEA-S to estriol. Since it is recently reported that P-450arom is considerably expressed in muscle tissues as well as fat tissues and that muscle tissue may be a major organ to produce estrogens in men and postmenopausal women, estriol production may be increased in the present patient's muscle. Although hypogonadism due to hypothalamus-pituitary disorders was sometimes reported, there have been no reports that suggest an increased estrogen production in skeletal muscles in mitochondrial encephalomyopathies. Recently, estrogen has been known to protect muscle fibers from oxidative damages due to exercise. Thus, it is of potential that estrogens increased locally in muscle tissues of the patients with mitochondrial encephalomyopathies protect muscle fibers from oxidative damage due to mitochondrial dysfunction.