The adenoma detection rate (ADR) is the most important quality metric for colonoscopy. Numerous factors are known to influence ADR. However, no data on the effect of monitor size on ADR exist. The aim of this study was to compare the ADR and polyp detection rate (PDR) achieved using 2 different-size video monitors (19-inch diagonal and 32-inch diagonal). In a single-center, prospective, randomized clinical trial, endoscopists were randomized on a daily basis to perform routine ambulatory colonoscopies with either a 32-inch diagonal or a 19-inch diagonal video monitor. The study was conducted between October 2013 and April 2014 in an outpatient center of a tertiary referral hospital. Fifteen endoscopists performed 1795 outpatient colonoscopies (mean age, 55 years; 56% women; screening, 56%). There was no substantial difference in baseline patient characteristics between the 2 arms. The overall ADR (27.4% vs 27.9%; P = .80) and PDR (32.8% vs 34.4%; P = .50) were not significantly different between the 32-inch and 19-inch monitor group, respectively. The findings were not significantly altered when stratified by indication, cecal intubation, bowel preparation, operator experience, and time of endoscopy as well as in a multivariable model that included these variables as potential confounders (all P > .05). Overall, the ADR and PDR for each individual endoscopist did not appear to be influenced by monitor size. The results of this trial do not support the notion that larger video monitors improve ADR. Future efforts to increase ADR should focus on other aspects of colonoscopy. (Clinical trial registration number: NCT01952418.).