The possible association between the risk of lung cancer in women and the levels of calcium (Ca) and magnesium (Mg) in drinking-water from municipal supplies was investigated in a matched, case-control study in Taiwan. All eligible female lung cancer deaths (3,532 cases) of Taiwan residents, from 2000 through to 2008, were compared with deaths from other causes (3,532 controls), and the levels of Ca and Mg in drinking-water of these residents were determined. Data on Ca and Mg 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 sex, year of birth, and year of death. The adjusted odd ratios were not statistically significant for the relationship between Ca levels in drinking-water and lung cancer in women. The adjusted odd ratios for female lung cancer deaths for those with higher Mg levels in their drinking-water, as compared to the lowest tertile, were 0.82 (95% CI = 0.72-0.93) and 0.80 (95% CI = 0.69-0.93), respectively. The results of the present study show that there is a significant trend toward a decreased risk of lung cancer in women with increasing Mg levels in drinking-water.