The acid, mutagenic compounds present in chlorinated drinking water have caused concern about the potential cancer risk of drinking-water mutagenicity. In this study, past exposure to drinking water mutagenicity was assessed for the years 1955 and 1970 in 56 Finnish municipalities, using the historical information on water quality and treatment. Cases of leukemia, lymphomas, and cancers of the liver, pancreas, and soft tissue were derived from the Finnish Cancer Registry for two periods: 1966-1976 and 1977-1989. Relative risk was estimated in an additive Poisson regression model, adjusting for age, gender, social class, urbanity, and time period. In an ordinary municipality that was supplied with mutagenic drinking water (3,000 net rev/l), the observed exposure-response relationship indicated a relative risk of 1.1-1.3 for lymphomas and 1.1-1.2 for pancreatic cancer, compared with municipalities in which nonmutagenic drinking water was consumed.