Endocrine functions were examined in 21 patients with mitochondrial myopathies presenting with chronic progressive external ophthalmoplegia and other additional neurological and multisystemic symptoms. Ten patients had the features of the Kearns-Sayre syndrome. Deletions of the mitochondrial DNA were found in 4 out of 5 patients examined. Fourteen patients, including 3 with deletions of the mitochondrial DNA, had various and often multiple endocrine abnormalities: 6 patients were of short stature, 3 had irregular menstrual cycles, 3 had undersized testicles, 5 showed an insufficient rise of growth hormone following the administration of growth-hormone-releasing hormone, 4 showed an insufficient rise in FSH after administration of gonadotropin-releasing hormone, 5 had manifest diabetes mellitus, 3 showed an impaired glucose tolerance, and 2 patients had subnormal serum levels of parathormone in combination with hypocalcaemia. One patient additionally had Klinefelter's syndrome with a kariotype 47, XXY and increased levels of FSH and LH, subnormal levels of testosterone and subnormal testicular volume. The occurrence of endocrine defects correlated with the duration of disease. The data demonstrate that endocrine abnormalities are frequently associated with mitochondrial myopathy, indicating that this multisystemic disease also involves various endocrine tissues.