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BACKGROUND AND AIMS Polypectomy technique has been shown to vary among colonoscopists and interval colorectal cancer (ICRC) may result from incomplete resection of an adenoma. Methods to monitor polypectomy quality and which size of polyps resected to monitor have not been well defined. The aim of this study was to compare the rate of metachronous adenoma attributable to incomplete resection in polyps 6 to 9 mm versus polyps 10 to 20 mm. METHODS The segmental metachronous adenoma rate attributable to incomplete resection (SMAR-IR) was calculated by subtracting the rate of metachronous neoplasia (MN) in segments without adenoma from segments with adenoma. The primary outcome of the study was the SMAR-IR in polyps 6 to 9 mm and 10 to 20 mm found on index colonoscopy, respectively. RESULTS In total 337 patients were included in the analysis; 146 patients with a tubular adenoma (TA) of 10 to 20 mm in size and 191 patients with a TA of 6 to 9 mm in size as the most advanced lesion. For cases in which an index TA 10 to 20 mm adenoma was resected, the segmental metachronous adenoma rate (SMAR) in segments with adenoma was 21.0% and in segments without adenoma 9.6%, so the SMAR-IR was 11.4% (95% CI, 4.5 - 18.3). For cases in which an index TA 6 to 9 mm was resected, the SMAR in segments with adenoma was 22.0% and in segments without adenoma 8.8%, so the SMAR-IR was 13.2% (95% CI, 7.2 - 19.4). Among 6 colonoscopists, the SMAR-IR ranged between 7.0% and 15.5% for polyps 6 to 20 mm. CONCLUSIONS Metachronous neoplasia rates in segments with a TA 10-20 mm and a TA 6-9 mm are higher than the MN rates in segments without index neoplasia. Incomplete resection of neoplasia appears to be a significant risk factor for metachronous neoplasia in 6-9 mm lesions as well as larger ones.