Background & Aims: Small colorectal polyps may be removed with cold snare polypectomy (CSP). Some of these polyps may contain unexpectedly advanced neoplasia. Thus, it is important to establish criteria for excision that will ensure that the CSP specimens are adequate for accurate histological assessment. We retrospectively investigated depth of excised small polyps and their vertical margins in patients who underwent CSP. Method: CSP-excised specimens of 376 small colorectal polyps were examined. We histologically evaluated negative tumor vertical margins and complete resection through the muscularis mucosae, which was defined as muscularis mucosae present under the tumor along more than 80% of its horizontal axis. We also evaluated the fragmentation of the retrieved specimens. Results: The mean size of the 376 polyps was 4.9 ±1.4 mm, and 25 polyps (6.6%) had unexpectedly advanced histology. Thirty-two lesions (8.5%) were fragmented. In 275 (79.9%) of the remaining 344 unfragmented polyps, muscularis mucosae resection was judged complete. Vertical margins were confirmed negative in 99.6% (274/275) of polyps that had complete muscularis mucosae resection, but in only 33.3% (23/69) of polyps with incomplete resection. In 79 polyps (21%) (32 fragmented specimens and 47 unfragmented specimens), including 5 polyps with advanced histology, negative vertical margins could not be confirmed. Sessile morphology and location in the cecum were independent risk factors for incomplete muscularis mucosae resection and fragmentation. Conclusion: Incomplete muscularis mucosae resection and fragmentation at retrieval are risk factors for inadequate histological evaluation of CSP-excised small colorectal polyps, especially for sessile polyps and polyps in the cecum. Key words: Cold snare polypectomy − colorectal neoplasms − colorectal polyps − colorectal polypectomy − advanced colorectal neoplasia. Abbreviations: CSP: cold snare polypectomy; CI: confidence intervals; MM: muscularis mucosae; NBI: narrow-band imaging; OR: odds ratio.