Background There are no guidelines for the recommended interval to the next examination after colonoscopy with suboptimal bowel preparation. Objective To identify factors associated with early repeat colonoscopy after initial examinations with suboptimal preparations and to measure adenoma miss rates in this context. Design Retrospective study. Setting Hospital-based endoscopy unit. Patients Bowel preparation quality was recorded in 12,787 patients. Results Of 12,787 colonoscopies, preparation quality was suboptimal (poor or fair) in 3047 patients (24%). Among these 3047 patients, repeat examination was performed in <3 years in 505 (17%). Factors associated with early repeat colonoscopy included lack of cecal intubation (odds ratio OR 3.62, 95% confidence interval CI, 2.50-5.24) and finding a polyp (OR 1.55, 95% CI, 1.17-2.07). Among 216 repeat colonoscopies with optimal preparation, 198 adenomas were identified, of which 83 were seen only on the second examination, an adenoma miss rate of 42% (95% CI, 35-49). The advanced adenoma miss rate was 27% (95% CI, 17-41). For colonoscopies repeated in <1 year, the adenoma and advanced adenoma miss rates were 35% and 36%, respectively. Limitations Single-center, retrospective study. Conclusion Although a minority of patients undergo early repeat examination after colonoscopies done with suboptimal bowel preparation, the miss rates for colonoscopies done with suboptimal bowel preparation were high, suggesting that suboptimal bowel preparation substantially decreases colonoscopy effectiveness and may mandate an early follow-up examination.