BACKGROUND & AIMS: The aim of this study is to evaluate the findings on optical colonoscopy (OC) after a positive CT colonography (CTC) exam and characterize the type of polyps seen on OC but not reported by CTC. METHODS: Over an 18-month period a total of 159 asymptomatic adults had polyps seen on computed tomography colonography examination and subsequently underwent planned therapeutic optical colonoscopy. The colonoscopists were aware of the findings on CT colonography prior to further evaluation of the colon. Characteristics of polyps and adenomas seen on subsequent optical colonoscopy but not seen or reported on CT colonography were examined. RESULTS: The adenoma miss rate for CT colonography overall was 18.9% (25/132) including 6.2% (4/65) for polyps >9 mm and 18.2% (8/44) for polyps 6-9 mm. Three of the adenomas >9 mm not seen on CTC were sessile, and two were found in patients with technically difficult CT colonography studies due to poor colonic distention. No adenomas with advanced pathology <6 mm were found on optical colonoscopy but not reported on CT colonography. False-positive CTC referral where no polyp was seen on colonoscopy was 5.0%. CONCLUSIONS: CT colonography has adenoma miss rates similar to miss rates historically found with optical colonoscopy, with most missed adenomas being <10 mm and sessile in shape.