Nitrate ingested from drinking water and diet can contribute to the endogenous formation of carcinogenic N-nitroso compounds. Epidemiologic studies of nitrate and cancer have reported mixed findings; a brief overview will be given. We present an updated analysis of The Iowa Women's Health Study (IWHS), which in 2001 reported an increased risk for bladder cancer associated with long-term exposure to nitrate in municipal water supplies. Our reanalysis included an additional six years of bladder cancer cases and exposure assessment for both drinking water and diet nitrate. Nitrate intake was calculated for a cohort of 16,541 Iowa women (ages 55-69 at 1986 baseline) who used the same municipal drinking water supply for >10 years. A total of 112 incident bladder cancers were diagnosed in the cohort through 2004. Nitrate levels in finished (post-treatment) drinking water for all municipal supplies and nitrate levels in raw (pre-treatment) source water for supplies using alluvial groundwater were used to calculate mean nitrate levels (1955-88). Dietary nitrate was estimated using a 126-item food frequency questionnaire. Dietary nitrate was not associated with bladder cancer risk. An increased bladder cancer risk was associated with the highest quartile of exposure to nitrate in drinking water (>2.23 mg/L nitrate-nitrogen, Relative Risk (RR) = 2.16, 95%CI 1.09-4.28); the risk was highest for women in the high water nitrate group who also had higher meat intake and lower vitamin C intake (RR = 4.17, 95%CI 1.46-11.88). Reanalysis of the IWHS showed an increase bladder cancer risk associated with long term exposure to drinking water nitrate levels below the regulatory limit. Other studies have found bladder cancer risk associated with exposure to trihalomethanes (THMs) and haloacetic acids (HAAs), common disinfection by-products (DBPs). We are estimating historical THM and HAA levels in Iowa municipal water supplies based in order to evaluate the separate contributions and possible interactions of nitrate and DBPs to bladder cancer risk. Recent studies on emerging nitrogenous DBPs (halonitromethanes, haloacetonitriles, N-nitrosodimethylamine) document cytotoxic and genotoxic effects in some mammalian species; therefore, research on drinking water exposures to these compounds in human populations is warranted.