We conducted a population-based case-control study of bladder cancer in Iowa in 1986–1989 to evaluate the risk posed by tapwater containing chlorination byproducts. We combined information about residential history, drinking water source, beverage intake, and other factors with historical data from water utilities and measured contaminant levels to create indices of past exposure to chlorination byproducts. The study comprised 1,123 cases and 1,983 controls who had data relating to at least 70% of their lifetime drinking water source. After we adjusted for potential confounders, we calculated odds ratios for duration of chlorinated surface water of 1.0 (referent), 1.0, 1.1, 1.2, and 1.5 for 0, 1–19, 20–39, 40–59, and ≥60 years of use. We also found associations with total and average lifetime byproduct intake, as represented by trihalomethane estimates. Positive findings were restricted to men and to ever-smokers. Among men, odds ratios were 1.0 (referent), 1.1, 1.3, 1.5, and 1.9, and among ever-smokers, 1.0, 1.1, 1.3, 1.8, and 2.2, after adjustment for intensity and timing of smoking. Among nonsmoking men and women, regardless of smoking habit, there was no association. Among men, smoking and exposure to chlorinated surface water mutually enhanced the risk of bladder cancer. The overall association of bladder cancer risk with duration of chlorinated surface water use that we found is consistent with the findings of other investigations, but the differences in risk between men and women, and between smokers and nonsmokers, have not been widely observed.