It is possible that there is a systematic geographical variation in the proportion of deaths from ischemic heart disease that occur suddenly, and that this variation may be causally related to water hardness or some other environmental factor. Three indices of sudden death have been examined; each showed a correlation with water hardness that was of similar sign and magnitude. A detailed examination of 1686 deaths occurring in residents of two cities which differ widely in the hardness of their water supply confirmed that in deaths ascribed to heart disease the proportion of sudden deaths was higher (by 20-30%) in the northern (soft-water) city than in the southern (hard-water) city.