The diagnostic considerations are broad for patients who clinically present with obscure abdominal pain. Acute appendicitis can be diagnosed according to the clinical signs and symptoms, yet this can often be extremely challenging. Difficulties particularly arise when the presentation is atypical. Investigations that may help include laboratory tests, non-invasive imaging and laparoscopy. We experienced 3 cases of acute appendicitis that were diagnosed by colonoscopy. The colonoscopy revealed a bulging of the appendix into the cecal lumen. The mucosa was erythematous and edematous with pus draining from the appendiceal orifice. These findings were suggestive of appendicitis, which was confirmed after surgical appendectomy. In our experience, making the colonoscopic diagnosis of appendicitis might be helpful for patients with atypical symptoms, but colonoscopy should not be used for the diagnosis in the beginning if appendicitis is initially suspected. (Korean J Gastrointest Endosc 2010;41:52-55)