The possible association between the risk of esophageal cancer and the levels of calcium and magnesium in drinking water from municipal supplies was investigated in a matched case-control study in Taiwan. All eligible esophageal-cancer deaths (2204 cases) of Taiwan residents from 1991 through 2000 were compared with a sample of deaths from other causes (2204 controls), and the levels of calcium and magnesium in the drinking water of these residents were determined. Data on calcium and magnesium levels in drinking water throughout Taiwan were obtained from the Taiwan Water Supply Corporation (TWSC). The control group consisted of people who died from other causes, and the controls were pair-matched to the cases by gender, year of birth, and year of death. Compared to those with magnesium levels below 7.0 mg/l, the adjusted odd ratios (95 per cent confidence interval (CI)) were 0.75 (0.62-0.91) for the group with water magnesium levels between 7.3 and 14.0 mg/l and 0.71 (0.57-0.88) for the group with magnesium levels of 14.3 mg/l or more. The adjusted odd ratios for the relationship between calcium levels in drinking water and esophageal cancer were not statistically significantly different from 1. The results of the present study show that there is a significant trend toward a decreased risk of esophageal cancer with increasing magnesium level in drinking water.