Keshan disease was an endemic cardiomyopathy in China. The very low selenium intake of local people was considered to be an important causal factor. The main pathological characteristics of this disease was multifo-cal necrosis and fibrous replacement of myocardium that was scattered throughout the wall of all chambers.Two patterns of myocardial necrosis, myofibrillar pattern and mitochondrial pattern were distinguished in electron microscopy. The myofibrillar pattern was characterized by myofibril segmentation. It agreed well with the contraction band necrosis described in light microscopy. It was mainly seen in acute Keshan heart and might be related to circulatory disorders. Mitochondrial pattern was identical with myocytolysis of conventional pathology. It represented the typical lesion of Keshan disease.Mitochondria showed early and conspicuous changes in involved myo-cytes. Myofibrillar damage seemed to be secondary to the mitochondrial injury in the development of myocytolysis.Histochemical studies revealed that the acid phosphatase activity was obviously increased in muscle fibers surrounding the necrotic foci, and the succinic dehydrogenase activity was greatly reduced in damaged myocardio-cytes.