Studies of the occurrence of bladder cancer in relation to exposure to chlorinated drinking water, in general, have found a small excess frequency of bladder cancer among consumers of chlorinated drinking water. Despite near consistent findings of a small positive association, interpretation of the association has been cautious because of the inability to control for potential confounding by unmeasured differences in surface and ground water, as the index frequently used for exposure to levels of chlorination in drinking water compares chlorinated surface water to unchlorinated ground water. The authors undertook a case-control study to collect detailed data on residential history that was obtained from telephone interviews with informants of 614 individuals who died of primary bladder cancer and 1,074 individuals who died of other causes. The study was based in an area where surface water has been disinfected with either chlorine or a combination of chlorine and ammonia (chloramine) since 1938.