Background: The prevalence of missed polyps in patients with inadequate bowel preparation on screening colonoscopy is unknown. Objective: To determine the prevalence of missed adenomas in average-risk patients presenting for screening colonoscopy who are found to have inadequate bowel preparation. Design: Retrospective chart review. Endoscopy and pathology reports were examined to determine the characteristics of polyps. Data from repeat colonoscopies were collected through 2010. Setting: Outpatient endoscopy center at an academic medical center. Patients: This study involved patients who underwent outpatient average-risk screening colonoscopy between 2004 and 2009 documented to have inadequate bowel preparation and who had colonoscopy to the cecum. Main Outcome Measurements: Initial adenoma detection rate and adenoma detection rate on follow-up examination. Results: Inadequate bowel preparation was reported on 373 patients, with an initial adenoma detection rate of 25.7%. Of 133 patients who underwent repeat colonoscopy, 33.8% had at least 1 adenoma detected, and 18.0% had high-risk states detected (<3 adenomas, 1 adenoma <1 cm, or any adenoma with villous features or high-grade dysplasia). Per-adenoma miss rate was 47.9%. Among patients with at least 1 adenoma on repeat colonoscopy, 31.1% had no polyps on initial colonoscopy; mean time between colonoscopies was 340 days. Among patients with high-risk states, 25.0% had no polyps seen on initial colonoscopy; mean time between colonoscopies was 271 days. Limitations: Retrospective design. Conclusion: Adenomas and high-risk lesions were frequently detected on repeat colonoscopy in patients with inadequate bowel preparation on initial screening colonoscopy, suggesting that these lesions were likely missed on initial colonoscopy.