Background Previous studies examining the incidence of colorectal cancer after polypectomy have provided discordant findings. The aim of this study was to compare the risk of colorectal cancer after adenoma removal in routine clinical practice with the risk in the general population.

Design Cohort study based on detailed data from a population-based registry that has collected all cases of both colorectal cancers and adenomas diagnosed in a clearly-defined population since 1976.

Setting French administrative area of Côte-d'Or (Burgundy).

Methods Residents of the area diagnosed for the first time with colorectal adenoma between 1990 and 1999 were included (n=5779). Initial and follow-up data until December 2003 were used to calculate the colorectal cancer standardised incidence ratio (SIR) and cumulative probabilities after adenoma removal.

Results After a median follow-up of 7.7 years, 87 invasive colorectal cancers were diagnosed whereas 69 cases were expected. Compared with the general population, the overall SIR was 1.26 (95% CI 1.01 to 1.56). The risk of colorectal cancer depended on the characteristics of the initial adenoma (SIR 2.23 (95% CI 1.67 to 2.92) for advanced adenomas and 0.68 (95% CI 0.44 to 0.99) for non-advanced adenomas). In cases of advanced adenomas, the SIR was 1.10 (95% CI 0.62 to 1.82) in patients with colonoscopic follow-up and 4.26 (95% CI 2.89 to 6.04) in those without. The 10-year cumulative probabilities of colorectal cancer were, respectively, 2.05% (95% CI 1.14% to 3.64%) and 6.22% (95% CI 4.26% to 9.02%).

Conclusions In routine practice, the risk of colorectal cancer after adenoma removal remains high and depends both on initial adenoma features and on colonoscopy surveillance practices. Gastroenterologists should encourage patients to comply with long-term colonoscopic surveillance.