We have studied the rules of the geographical epidemiology. An article publishedin Acta Geographica Sinica Vol. 34, No. 2, P. 85, June, 1979, illustrates that the distri-bution of the Keshan disease is very orderly, and is closely related to the natural envi-ronment. The Keshan disease is mainly distributed in the area with the temperate (orwarm temperate) forest and forest-steppe soils as axis. It forms a wide belt runningfrom the Xorth-East to the South-West in our country. While the typical steppe-desertregions to the north West and the typical yellow-red soil regions to the South-East arcdisease-free. Thus, there are three belts in our country: one disease belt and two disease-free belts. The disease belt located in the middle, while the two others lie to its twosides. We think that this case might be concerned with the chemical geographical en-vironment.We explored the relationship between Keshan disease and the chemical factors of theenvironment. 1638 samples of various food grains from the main different regionsthroughout the country are collected. Seventeen elements are analysed, It is found thatthe selenium content in the food grain is closely related to the Keshau disease, namely,the difference of the selenium content in the food grain between the disease belt andthe disease-free belt is very significant. The average selenium content in the main foodgrains of the disease belt is less than about 0.020 PPm, while in the disease-free beltsthe content of selenium is much higher. The selenium content in most of the food grainssamples for the disease belt is less than 0.025 PPm, and for the disease-free belt more than0.040 PPm. The value 0.040 PPm might be considered as the reliable threshold value ofthe food grain selenium dividing the regions with and without disease.Hence, the selenium in natural environment has obvious regional differentiation.The Keshan disease occurs only where the grain selenium content is low. Then we mayconsider the low selenium environment as an important factor causing keshan disease.Our studies provide a sound geoscientifical basis for the cause of kashan disease.