The relationship between mortality from breast cancer and the levels of calcium and magnesium in drinking water was examined using an ecological design. The study area consisted of 252 municipalities in Taiwan. Data on the levels of calcium and magnesium in drinking water were collected from the Taiwan Water Supply Corporation (TWSC). The age-standardized mortality rate (ASR) for breast cancer (1982?1991) was compared among municipalities with different levels of magnesium and calcium in drinking water. Weighted multivariate regression analysis was used, and after adjusting for fertility rates and urbanization, there was a significant inverse relationship between the levels of calcium and magnesium in drinking water and risk of death from breast cancer.