The objectives of this study were to (1) examine the relationship between nitrate levels in public water supplies and risk of death from gastric cancer and (2) determine whether calcium (Ca) and magnesium (Mg) levels in drinking water might modify the effects of nitrate on the risk of gastric cancer development. A matched cancer case-control study was used to investigate the relationship between the risk of death attributed to gastric cancer and exposure to nitrate in drinking water in Taiwan. All deaths due to gastric cancer in Taiwan residents from 2006 through 2010 were obtained from the Bureau of Vital Statistics of the Taiwan Provincial Department of Health. Deaths from other causes served as controls and were pair-matched to cancer cases by gender, year of birth, and year of death. Information on the levels of nitrate-nitrogen (NO(3)-N), Ca, and Mg in drinking water were collected from Taiwan Water Supply Corporation (TWSC). The municipality of residence for cancer cases and controls was presumed to be the source of the subject's NO(3)-N, Ca, and Mg exposure via drinking water. Relative to individuals whose NO(3)-N exposure levels were <0.38 ppm, the adjusted odds ratio (OR) and 95% confidence interval (CI) for gastric cancer occurrence was 1.16 (1.05-1.29) for individuals who resided in municipalities served by drinking water with a NO(3)-N exposure ≥ 0.38 ppm. There was apparent evidence of an interaction between drinking water NO(3)-N levels and low Ca and Mg intake via drinking water. Our findings showed that the correlation between NO(3)-N exposure and risk of gastric cancer development was influenced by Ca and Mg levels in drinking water. This is the first study to report effects modification by Ca and Mg intake from drinking water on the relationship between NO(3)-N exposure and risk of gastric cancer occurrence. Increased knowledge of the mechanistic interactions between Ca, Mg, and NO(3)-N in reducing risk of gastric cancer development will aid in public policy decisions and setting threshold standards.