Earlier studies have shown an association between cold climate and high coronary mortality. The aim of this study was to elucidate in more detail the effect of drinking water hardness on this association. Drinking water parameters were obtained from 259 municipalities in Sweden over a 10-year period. During the same period a 'cold index' was calculated for each municipality. Standardized coronary mortality rates in men aged 40-64 years were used. A negative association was found between water hardness and coronary mortality (coefficient of determination k = 0.080). A much stronger association was found between coronary mortality and cold index (D = 0.389). When coronary mortality was explained by both water hardness and cold index, the association was not significantly strengthened (D = 0.391). Water hardness was negatively correlated with cold index (r = 0.55). We conclude that the association between cold climate and coronary mortality was stronger than that between drinking water hardness and coronary mortality, and that it persisted after stratification for water hardness.