The purpose of this study is to describe our experience with cases of false negative findings at conventional colonoscopy (CC) that were identified by CT colonography (CTC). Conventional colonoscopy (CC) is the universally accepted gold-standard technique for the diagnosis of colonic polyps and cancers, however occasionally this method can generate false negative findings. We present examples of false negatives at CC, correctly identified by CT colonography (CTC), and later confirmed at a second endoscopy, describing the reasons of false negative, when possible.