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Background: The wide range of R0 resection rates (R0RR) and incomplete resection rates (IRR) observed with conventional cold snare polypectomy (CCSP) emphasizes the necessity for technique enhancement. The COLDWATER study aimed to compare underwater cold snare polypectomy (UCSP) to CCSP for 5-10-mm colorectal polyps, focusing on comprehensive histopathological evaluation, efficacy, and safety., Methods: This was a randomized, single-blind, controlled trial comparing UCSP to CCSP for non-pedunculated colorectal polyps of size 5-10 mm. The primary outcome was to report differences in the muscularis mucosa resection ratio. The secondary outcomes focused on differences in depth of excision, R0-RR, IRR, en bloc resection rate, adverse events, and recurrence rate., Results: The COLDWATER study found higher muscularis mucosa resection in UCSP (81.72±62.81% vs. CCSP: 72.33±22.33%, P=0.003) with comparable submucosa presence (UCSP: 16.6%, CCSP: 12.5%, P=0.25). UCSP showed better outcomes regarding IRR (3.5% vs. 8.5%, P=0.05) and en bloc resection (98% vs. 93.5%, P=0.04). In CCSP, expert endoscopists achieved higher R0RR than non-experts, while UCSP showed no significant difference in R0RR across endoscopist's experience levels., Conclusions: UCSP achieves a more extensive excision of the muscularis mucosa compared to CCSP, even though it does not attain a deeper excision. Additionally, UCSP shows a higher en bloc resection rate, with lower rates of IRR, and emerges as a promising technique for training inexperienced endoscopists in polypectomy, given its experience-independent success in achieving R0 resection., Competing Interests: Conflict of Interest: None, (Copyright: © 2024 Hellenic Society of Gastroenterology.)