A 7½‐year‐old girl with hypermethioninemia, myopathy, and mental deficiency (IQ = 65) is described. The increased methionine was not associated with deficiency of methionine adenosyltransferase, which was normal or increasedin liver, muscle, erythrocytes, and cultured fibroblasts. Methionyl‐tRNA synthetase in fibroblasts was normal. The hypermethioninemia and a concurrently increased blood S‐adenosylmethionine declined on a diet low in methionine. There was a diffuse, symmetrical, moderate proximalmuscle weakness, but muscle atrophy was not discernible, and the deep tendon reflexes were hypoactive but obtainable. Electromygraphic abnormalites were not detected. Electron microscopy of muscle revealed 3 to 6 small myelin figures in the region of the I band in nearly every fiber, with occasional myelin figures at other sites also. These myelin figures were more numerous and smaller than those seen accompanying nonspecific and may reflect a more specific pathological change. Electron microscopy of liver revealed three nonspecific lesions in all hepatocytes: (1) numerous megamitochondria with crystalloid deposit in the matrix; (2) increased numbers of small vesicles of smooth endoplasmic reticulum; and (3) loss of plasma membrane microvilli, with extensive bleb for formation and shedding of eytoplasm into Disse's space.