Background and Aims Limited data are available to investigate the impact of index adenoma size on the risk of metachronous advanced adenomas. Our goal was to examine the impact of having small (5-9 mm) versus diminutive (<5 mm) adenomas on the future risk of advanced adenomas within the categories for polyps <1cm currently used in the United States: 1 to 2 and 3 or more tubular adenomas. Methods We included data from individuals participating in the statewide, population-based New Hampshire Colonoscopy Registry (NHCR). Groups were based on index findings: (1) 1 to 2 adenomas <5 mm (both diminutive), (2) 1 to 2 adenomas <1 cm (one or both small), (3) 3 to 10 adenomas <5 mm (all diminutive), (4) 3 to 10 adenomas <1 cm (one or more small), and (5) advanced adenomas (AA). AAs were defined as adenomas >1cm or those with villous elements or high-grade dysplasia and colorectal cancer (CRC). Outcomes were the absolute and adjusted risk of metachronous advanced adenomas. Covariates included age, sex, body mass index, family history of CRC, lifestyle factors, presence of serrated polyps, and time since the index examination. Results After adjusting for the covariates, we observed that having 1 to 2 adenomas with at least one 5 to 9 mm adenoma (adjusted odds ratio [AOR], 1.54; 95% CI, 1.12-2.11), 3 to 10 diminutive adenomas (AOR, 1.75; 95% CI, 1.03-2.95), 3 to 10 adenomas <1 cm (1 or more small) (AOR, 2.14; 95% CI, 1.39-3.29) or advanced adenomas (AOR, 2.77; 95% CI, 2.05-3.74) were associated with an increased risk for metachronous AA as compared with having 1 to 2 diminutive adenomas. A further stratification of Group 2 observed that those with exactly 2 small adenomas had an absolute risk of future AA of 7.6% (11/144) (95% CI, 4.3%-13.2%), higher than the absolute risk in the 1 to 2 diminutive polyp group, and similar to the risk for 3 to 10 adenomas 8.2 (95% CI, 5.4-11.9). Conclusions For individuals with 1 to 2 adenomas <1 cm, having at least 1 small adenoma increased the metachronous risk of AA compared to having only diminutive adenomas. Furthermore, the subset with 2 small adenomas had a risk of future AA similar to the risk for 3 to 10 adenomas. These data suggest that individuals with at least 1 small adenoma may be at higher risk for future advanced adenomas and thus require closer follow-up than those with only diminutive adenomas. These data may be valuable to guideline committees for the creation of future surveillance recommendations. Background Colorectal cancer (CRC) is the second most common cause of death from cancer in the United States 1 , despite being a preventable disease. The key to prevention lies in delivery of appropriate and timely screening and surveillance colonoscopies, and development of appropriate guideline recommendations for prevention requires evidence to inform those guidelines. Colonoscopy is the most common form of CRC screening in the United States, and surveillance colonoscopies, which are follow-up colonoscopies in individuals found to have