Mutagenicity of drinking water due to chlorination byproducts may pose increased risks of bladder, kidney, and stomach cancer. The risks of rectum and colon cancers were not thought to be raised by water mutagenicity. A retrospective study of past exposure to drinking water mutagenicity was conducted for 56 municipalities in Finland from 1955 to 1970. The study was possible because of the existence of data from these years and since then on water quality, rates of cancer, and variations in drinking water mutagenicities. The water mutagen MX may be a possible cause of increased risk of bladder and kidney cancer, as it may become stored in higher concentrations in these locations than in other tissues. Level of mutagenicity may correlate more strongly with risk of cancer than either drinking water chlorination or mutagenicity per se.