Background: Evidence on the association between colorectal cancer and exposure to disinfection by-products in drinking water is inconsistent. Objectives: We assessed long-term exposure to trihalomethanes (THMs), the most prevalent group of chlorination by-products, to evaluate the association with colorectal cancer. Methods: A multicentre case-control study was conducted in Spain and Italy in 2008-2013. Hospital-based incident cases, population-based (Spain) and hospital-based (Italy) controls were interviewed to ascertain residential histories, water type consumed in each residence, frequency and duration of showering/ bathing, and major recognized risk factors for colorectal cancer. We estimated adjusted odds ratios (OR) for colorectal cancer in association with quartiles of estimated average lifetime THM concentrations in each participant’s residential tap water (μg/L, from age 18 to two years before the interview) and estimated average lifetime THM ingestion from drinking residential tap water (μg/day). Results: Subjects analyzed were 2047 cases and 3718 controls. Median values (ranges) for average lifetime residential tap water concentrations of total THMs, chloroform, and brominated THMs were 30 (0-174), 17 (0-63), and 9 (0–145) μg/L, respectively. Total THM concentration in residential tap water was not associated with colorectal cancer (OR=0.92, 95%CI: 0.66-1.28 for highest vs. lowest quartile), but chloroform concentrations were inversely associated (OR=0.31, 95%CI: 0.24-0.41 for highest vs. lowest quartile). Brominated THMs concentrations showed a positive association among men at the highest vs. lowest quartile (OR=1.43, 95%CI: 0.83-2.46). Patterns of associations were similar for estimated average THM ingestion through residential water consumption. Conclusions: We did not find clear evidence of an association between detailed estimates of lifetime total THM exposures and colorectal cancer in our large case-control study population. Negative associations with chloroform concentrations and ingestion suggest differences among specific THMs, but these findings need confirmation in other study populations.