Background: Chlorination is a widely used method for disinfection of drinking water and although effective in reducing microbial growth it may introduce undesired reactive chemicals into the water. Several by-products are considered genotoxic and carcinogenic, something that has been clearly verified by animal studies. Calcium and Magnesium are minerals naturally present in drinking water. The level of evidence of an effect of both minerals on the development of cardiovascular disease is high, however, the relevance of the concentrations in drinking water is not known. Hypothesis: Chlorination by-products pre-dispose individuals to develop bladder and colorectal cancer. Calcium increase risk of cardiovascular events whereas magnesium has a protective effect on disease risk. Aim: To assess whether chlorination of drinking water predispose individuals to develop bladder or colorectal cancer and to assess if calcium and magnesium affects the risk of cardiovascular disease. Method: The study population will be the male and female participants of SMC and COSM. Retrospective information on concentrations of trihalomethanes and Mg and Ca in municipal drinking water will be collected from Vattentäktsarkivet and the Swedish Water and Waste water Association, together with historical information on the drinking water production methods used. Localities within the SMC and COSM counties will be separated into categories as follows: high, medium, low or no chlorination and high or low calcium/magnesium. Cancer and cardiovascular outcomes will be ascertained via the linkage to national patient registry. Associations between exposures and outcomes will be assessed by fitting cox proportional hazard regression models, adjusting for potential covariates