Background: The pediatric colonoscope is superior to the standard colonoscope in some patients with a fixed,angulated colon. A colonoscope thinner than the pediatric one is thought to out- perform even the pediatric colonoscope in such cases. This study was conducted to assess the efficacy of an ultrathin colonoscope, 9.8 mm in diameter, comparing it with pediatric and standard colonoscopes. Methods: Three types of colonoscopes were used: ultrathin, pediatric, and standard. A total of 287 consecutive patients were assigned to three groups: ultrathin (n = 94), pediatric (n = 98), and standard (n = 95). First assessment was the ratio of cecal intubation and the reasons for unsuccessful colonoscopy. The second was the time to cecal intubation. Results: Cecal intubation rates were not different among three groups (96% in each). The main reasons for failed colonoscopy were looping in the ultrathin group, and angulation or stricture in the pediatric and standard groups. Mean time to the cecum was slightly longer in the ultrathin group (6.5 minutes) than the pediatric group (5.6 minutes) and standard group (6.1 minutes), but there were no significant differences among three groups. Conclusion: Colonoscopy with the ultrathin colono- scope was as successful as with the pediatric and standard colonoscopes. The advantage of the ultrathin colonoscope might be notable in cases with stricture or severe angulation. Background: The pediatric colonoscope is superior to the standard colonoscope in some patients with a fixed, angulated colon. A colonoscope thinner than the pediatric one is thought to out- perform even the pediatric colonoscope in such cases. This study was conducted to assess the efficacy of an ultrathin colonoscope, 9.8 mm in diameter, comparing it with pediatric and standard colonoscopes. Methods: Three types of colonoscopes were used: ultrathin, pediatric, and standard. A total of 287 consecutive patients were assigned to three groups: ultrathin (n = The second was the time to cecal intubation. Results: Cecal intubation rates were not different among the three groups (96% in each). The main reasons for failed colonoscopy were looping in the ultrathin group, and angulation or stricture in the pediatric and standard groups. Mean time to the cecu m was slightly longer in the ultrathin group (6.5 minutes) than the pediatric group (5.6 minutes) and standard group (6.1 minutes), but there were no significant differences among three groups. Conclusion: Colonoscopy with the ultrathin colono- scope was as successful as with the pediatric and standard colonoscopes. The advantage of the ultrathin colonoscope might be not in in cases with stricture or severe angulation.