Background.Nitrate is a precursor compound in the formation of N-nitroso compounds, most of which are potent animal carcinogens. N-nitroso compounds and their precursors have not been extensively evaluated as bladder cancer risk factors.Methods.We conducted a population-based case-control study of bladder cancer in Iowa. Cases were men and women newly diagnosed with bladder cancer in 1986–1989. Nitrate data for Iowa public water supplies were sparse before the 1960s. To reduce misclassification by unknown nitrate levels, we included only those who used public supplies with nitrate data for 70% or more of their person-years since 1960 (808 cases, 1259 controls).Results.Among controls, the median average nitrate level for their Iowa residences with public water supplies was 1.3 mg/liter nitrate-nitrogen (interquartile range = 0.6–3.0). After adjustment for confounders, we found no increased risk of bladder cancer with increasing average nitrate levels in drinking water; the highest quartile odds ratio for women was 0.8 (95% confidence interval = 0.4–0.8), and for men 0.5 (0.4–0.8). We observed no association among those with high water nitrate exposure (>median) and low (<median) vitamin C intake compared with those who had low water nitrate and high vitamin C intake.Conclusions.Our data suggest that long-term exposure to nitrate in drinking water at levels in this study (90th percentile 5.5 mg/liter nitrate-nitrogen) is not associated with risk of bladder cancer.