Introduction: Rates of colorectal cancer (CRC) in adults under age 50 is increasing, leading the American Cancer Society (ACS) to recommend lowering the initial screening age to 45. Little is known about best screening practices in this population. Underwater insertion (UW) has been shown to improve polyp detection rates among older adults. However, its effect on polyp detection in the young is less well understood. Objective: Assess the effect of underwater insertion on polyp detection among young adults undergoing colonoscopy. Methods: Colonoscopy quality data was prospectively collected at our single, tertiary referral center beginning in June 2012. We analyzed all colonoscopies performed between Aug 2014 and June 2018 in patients age 19-49, inclusive of all indications. Procedures were categorized based on insertion type: air/CO2-insufflated (AI) or underwater (UW). The two groups were compared with respect to polyp detection rates, procedural times, and cecal intubation rates. Significance was determined using chi-square test and unpaired T-test. Results: 583 colonoscopies were included in final analysis, including 168 AI and 415 UW procedures. The most common indications for colonoscopy were personal history, abdominal pain, and bleeding. Bowel prep scores were higher in UW group (7.7 vs. 8.1, p=0.02). UW was associated with a significant increase in ADR over AI (5.2% vs. 16.4%, p<0.0001). Overall PDR also improved with UW, though these results did not reach significance (24.4% vs. 31.0%, p=0.08). Similar significant increases in ADR were seen on subgroup analysis by gender in both females (2.1% vs. 14.6%, p<0.0001) and males (7.5% vs. 18.3%, p=0.01). UW also improved detection of both polyps in the left colon (2.1% vs. 10.5, p=0.0002) and right colon (4.3% vs. 9.8%, p=0.03). Procedural times were marginally longer with UW, though this was not significant (see table 3). Conclusion: The ACS has recommended expanding the target CRC screening population to include younger patients. UW has been shown to significantly improve polyp detection and bowel prep scores, and reduce sedation need in the age 50 and older screening cohort. Our results suggest these benefits extend to younger patients. Our results are limited by the retrospective analysis and non-screening indications for procedures. Further prospective studies are needed to assess the role of UW, as well as other techniques and devices, on polyp detection in younger patients.165_A Figure 1. Baseline characteristics of patient population165_B Figure 2. Polyp detection rates with air-insufflation vs. underwater insertion165_C Figure 3. Procedure times and Boston Bowel Prep Scores with air insufflation vs. underwater insertion