Background: Population-based screening to prevent colorectal cancer (CRC) death is effective, but the effectiveness of postpolypectomy surveillance is unclear. Objective: To evaluate the additional benefit in terms of costeffectiveness of colonoscopy surveillance in a screening setting. Design: Microsimulation using the ASCCA (Adenoma and Serrated pathway to Colorectal CAncer) model. Data Sources: Dutch CRC screening program and published literature. Target Population: Asymptomatic persons aged 55 to 75 years without a prior CRC diagnosis. Time Horizon: Lifetime. Perspective: Health care payer. Intervention: Fecal immunochemical test (FIT) screening with colonoscopy surveillance performed according to the Dutch guideline was simulated. The comparator was no screening or surveillance. FIT screening without colonoscopy surveillance and the effect of extending surveillance intervals were also evaluated. Outcome Measures: CRC burden, colonoscopy demand, lifeyears, and costs. Results of Base-Case Analysis: FIT screening without surveillance reduced CRC mortality by 50.4% compared with no screening or surveillance. Adding surveillance to FIT screening reduced mortality by an additional 1.7% to 52.1% but increased lifetime colonoscopy demand by 62% (from 335 to 543 colonoscopies per 1000 persons) at an additional cost of €68 000, for an increase of 0.9 life-year. Extending the surveillance intervals to 5 years reduced CRC mortality by 51.8% and increased colonoscopy demand by 42.7% compared with FIT screening without surveillance. In an incremental analysis, incremental costeffectiveness ratios (ICERs) for screening plus surveillance exceeded the Dutch willingness-to-pay threshold of €36 602 per life-year gained. Results of Sensitivity Analysis: When using a parameter set representing low colorectal lesion prevalence or when colonoscopy costs were halved or colorectal lesion incidence was doubled, screening plus surveillance became cost-effective compared with screening without surveillance. Limitation: Limited data on FIT performance and background CRC risk in the surveillance population. Conclusion: Adding surveillance to FIT screening is not costeffective based on the Dutch ICER threshold and substantially increases colonoscopy demand. Extending surveillance intervals to 5 years would decrease colonoscopy demand without substantial loss of effectiveness. Primary Funding Source: Alpe d'HuZes, Dutch Cancer Society, and Stand Up To Cancer.