Following observations by Kobayashi in Japan, which indicated a correlation between the acidity but not the hardness of river water and the death rate from cerebral hemorrhage, Schroeder1 made 6 studies of the drinking water and ageadjusted death rates by states. Long-noted regional differences in death rates from cardiovascular diseases can hardly be explained on the basis of racial background, diet, food sources, or environmental stresses. An environmental factor to which everyone is exposed is the local potable water supply. It was found that a significant negative correlation existed between the hardness of the water and the death rates due to cardiovascular disease, especially those secondary to atherosclerosis or hypertension. The greater the hardness, the lower the death rate from those causes. There was no such correlation with noncardiovascular diseases. When similar studies were made of 163 cities the same correlation was observed with death rates from coronary occlusion.