A relation between water hardness and cardiovascular death has been shown in previous studies. In this case-control study, we investigated the levels of magnesium and calcium in drinking water and death from acute myocardial infarction among women. The study population encompassed 16 municipalities in southern Sweden. Cases were women who had died from acute myocardial infarction between the ages of 50 and 69 years during 1982-1993 (N = 378), and controls were women who had died from cancer (N = 1,368). We obtained magnesium and calcium concentrations of the individual water sources. We divided the subjects into quartiles and found that odds ratios (ORs) were lower at higher levels of both magnesium and calcium. For the quartile with the highest magnesium levels (> or =9.9 mg/liter), the OR adjusted for age and calcium was 0.70 (95% confidence interval = 0.50-0.99). For calcium, the adjusted OR for the quartile with the highest level (> or =70 mg/liter) was 0.66 (95% confidence interval = 0.47-0.94). The results suggest that magnesium and calcium in drinking water are important protective factors for death from acute myocardial infarction among women.