BACKGROUND: Adenoma detection rate (ADR) has become the most important quality indicator for colonoscopy. OBJECTIVE: The aim of this study was to investigate which modifiable factors, directly related to the endoscopic procedure, influenced the ADR in screening colonoscopies. DESIGN: Observational, nested study. SETTING: Multicenter, randomized, controlled trials. PATIENTS: Asymptomatic people aged 50 to 69 years were eligible for a multicenter, randomized, controlled trial designed to compare colonoscopy and fecal immunochemical testing in colorectal cancer screening. A total of 4539 individuals undergoing a direct screening colonoscopy were included in this study. INTERVENTION: Colonoscopy. MAIN OUTCOME MEASUREMENTS: Bowel cleansing, sedation, withdrawal time in normal colonoscopies, and cecal intubation were analyzed as possible predictors of adenoma detection by using logistic regression analysis, adjusted for age and sex. RESULTS: In multivariate analysis, after adjustment for age and sex, factors independently related to the ADR were a mean withdrawal time longer than 8 minutes (odds ratio [OR] 1.51; 95% CI, 1.17-1.96) in normal colonoscopies and split preparation (OR 1.26; 95% CI, 1.01-1.57). For advanced adenomas, only withdrawal time maintained statistical significance in the multivariate analysis. For proximal adenomas, withdrawal time and cecal intubation maintained independent statistical significance, whereas only withdrawal time longer than 8 minutes and a <10-hour period between the end of preparation and colonoscopy showed independent associations for distal adenomas. LIMITATIONS: Only endoscopic variables have been analyzed. CONCLUSION: Withdrawal time was the only modifiable factor related to the ADR in colorectal cancer screening colonoscopies associated with an increased detection rate of overall, advanced, proximal, and distal adenomas.