Objective Post-polypectomy colonoscopy surveillance aims to prevent colorectal cancer (CRC). The 2002 UK surveillance guidelines define low-, intermediate-, and high-risk groups, recommending different strategies for each. Evidence supporting the guidelines is limited. We examined CRC incidence and effects of surveillance on incidence among each risk group. Design Retrospective study of 33,011 patients who underwent colonoscopy with adenoma removal at 17 UK hospitals, mostly (87%) from 2000–2010. Patients were followed-up through 2016. Cox regression with time-varying covariates was used to estimate effects of surveillance on CRC incidence adjusted for patient, procedural, and polyp characteristics. Standardised incidence ratios (SIRs) compared incidence with that in the general population. Results After exclusions, 28,972 patients were available for analysis; 14,401 (50%) were classed as low-risk, 11,852 (41%) as intermediate-risk, and 2719 (9%) as high-risk. Median follow-up was 9.3 13years. In the low-, intermediate-, and high-risk groups, CRC incidence per 100,000 person-years was 14140 (95%CI 122–162), 221 (195–251), and 366 (295–453), respectively. CRC incidence was 40–50% lower with a single surveillance visit than with none: hazard ratios were 0.56 (0.39–0.80), 0.59 (0.43–0.81), and 0.49 (0.29–0.82) in the low-, intermediate-, and high-risk groups, respectively. Compared with the general population, CRC incidence without surveillance was similar among low-risk (SIR 0.86, 0.73–1.02) and intermediate-risk (1.16, 0.97–1.37) patients, but higher among high-risk patients (1.91, 1.39–2.56).20Conclusion Post-polypectomy surveillance reduces CRC risk. However, even without surveillance, CRC risk in some low-risk and intermediate-risk patients is no higher than in the general population. These patients could be managed by screening rather than surveillance.