['Colorectal polyps can be missed even during a meticulous colonoscopy by an experienced colonoscopist. The aim of this study was to investigate patient-, procedure-, and polyp-related factors affecting the miss rate of polyps on colonoscopy.', 'From March 2001 to February 2006, a total of 285 patients undergoing screening colonoscopy followed within 2 months by colonoscopy for polypectomy were enrolled. The main outcome measurements were factors associated with miss rates of polyp and the conditions for reducing missed polyps on colonoscopy.', 'From the 285 colonoscopy pairs, a total of 1326 polyps were found. Overall, 350 of the 1326 polyps were missed (26.4%). Among those, 59 polyps (4.4%) were only seen on the first colonoscopy and 291 polyps (21.9%) were found only at the second colonoscopy. Patient-related factors were male sex (Odds Ratio (OR) 2.11, 95% Confidence Interval (CI) 1.21-3.70) and older age (OR 2.51, 95% CI 1.48-4.30). Procedure-related factors were colonoscopy by clinical fellows (OR 2.20, 95% CI 1.02-4.84) and delayed insertion time (OR 4.10, 95% CI 2.14-7.86). Polyp-related factors were more than four numbers of polyps (OR 4.48, 95% CI 1.91-10.5). However, a large polyp was less frequently missed on colonoscopy (OR 0.10, 95% CI 0.05-0.19).'] Do influence of the insertion time and number of polyps on miss rate in colonoscopy?