A number of studies carried out in other countries have shown the existence of negative correlations between coronary mortality and the hardness of water supplies. Data on water hardness throughout Taiwan have been collected from Taiwan Water Supply Corporation (TWSC) to examine the ecological correlation between deaths from coronary disease (1981–1990) and total hardness in drinking water. Analysis demonstrates a significant negative association between drinking water hardness and coronary mortality. After adjustment for the urbanization index, coronary mortality in municipalities with soft water was estimated to be 9.6% higher than that in municipalities with hard water. The weighted multivariate-adjusted regression coefficient indicated a decrease of 0.053 in standardized mortality ratios (SMRs) for every 100 mg/L increase in total hardness in drinking water after allowing for the urbanization condition.