Disinfectant byproducts (DBPs) haloacetic acids (HAA5) and total trihalomethanes (TTHM) used in community water systems (CWS) are potential bladder and colorectal carcinogens. Studies neglect to account for how exposures may differ within counties based on population served (PS) by CWS, which provides estimates of individual exposure. CWS concentration data for HAA5 and TTHM with PS were obtained for eight US states. These were summed to create additive DBP exposure and aggregated to the county-level to construct three cumulative county-level HAA5, TTHM, and additive DBP exposure metrics: unadjusted for PS; controlled for PS (as a covariate); and accounted for PS (concentration multiplied by proportion of PS by county population). Poisson regressions estimated incidence rate ratios (IRR) and 95% confidence intervals for associations between colorectal and bladder cancer and DBP, HAA5, and TTHM exposure tertiles, adjusting for potential confounders. For unadjusted DBP tertiles (<331 parts per billion (ppb), 331-732 ppb, >732ppb), the IRRs for colorectal cancer were 1.27 (1.22,1.32) and 1.58 (1.53,1.64) for tertiles 2-3, compared to <331 ppb, the lowest exposure category. For DBP tertiles controlled by PS, the IRRs were 1.27 (1.22,1.32) and 1.59 (1.54,1.65) for tertiles 2-3, compared to <331 ppb. After accounting for PS (<178 ppb, 178-502 ppb, >502 ppb), the IRRs were 0.92 (0.88,0.96) and 1.46 (1.41,1.51) for tertiles 2-3, compared to <178 ppb. Associations between colorectal cancer and HAA5 and TTHM tertiles were similar to the DBP tertiles. Associations between bladder cancer and HAA5, TTHM, and DBP tertiles were also similar to colorectal. Differing measures of PS by CWS in measured drinking water HAA5, TTHM, and DBP concentrations resulted in positive but varied associations to colorectal and bladder cancer. Thus, PS may need to be considered when developing drinking water exposure measures. This abstract does not reflect EPA policy.