['Few data are available on the influence of a colonoscope length for trainees, which could affect both the duration of training and colonoscopy quality. We conducted this study to validate which scope needs more duration for training to reach technical competence and to shows better quality indicators during diagnostic colonoscopy.', 'We conducted a prospective randomly assigned study from April 2010 to February 2011 at Asan Medical Center. Among the 1329 patients enrolled, 1200 colonoscopies were analyzed. We compared cecal intubation rate, adenoma detection rate, cecal intubation time (<20 min), and withdrawal time between the trainees using the intermediate-length colonoscope and those using long-length colonoscope.', 'Trainees who used the long-length colonoscope showed a higher overall cecal intubation rate (88.2% vs. 81.0%, p = 0.001) and adenoma detection rate (49.7% vs. 34.2%, p < 0.001) than those using the intermediate-length colonoscope. The successful cecal intubation rate improved significantly and reached the requisite standard of competence (>90%) after 90 procedures in the long-length colonoscope group. However, the trainees using the intermediate-length colonoscope reached the requisite standard of competence after 150 procedures. Logistic regression analysis revealed that prolonged cecal intubation was associated with the use of the intermediate-length colonoscope, poor colon preparation, a small number of esophagastroduodenoscopy or sigmoidoscopy procedures conducted, and pain during procedures.'] Does intermediate-length colonoscope need more training duration than long-length colonoscope?