We investigated endocrine function in patients with mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS), myoclonus epilepsy associated with ragged-red fibers (MERRF), and chronic progressive external ophthalmoplegia (CPEO). Hypothalamic-pituitary function was impaired in all three patients with MELAS or MERRF, but none of four with CPEO. A MELAS patient with dwarfism and impaired adolescent development had decreased growth hormone, luteinizing hormone (LH), and follicle-stimulating hormone (FSH). A MERRF patient had emaciation and low adrenocorticotropin. A patient with mitochondrial encephalomyopathy transitional between MELAS and MERRF showed delayed, blunted LH and FSH response to LH-releasing hormone stimulation. We concluded that patients with mitochondrial encephalomyopathies, especially MELAS or MERRF, are likely to have hypothalamic-pituitary dysfunction.