Geographic variations in the hardness of water seem to parallel the geographic variations in certain cardiovascular diseases (Anderson and others, 1969; Biorck and others, 1965; Schroeder, 1966). Winton and McCabe (1970) have provided a recent review of the problem. The map, presented here, showing calcium-carbonate hardness of municipal raw-water sources in the conterminous United States has been compiled to provide data for further examination of correlations between water hardness and cardiovascular diseases.