We prospectively studied the colonoscopic miss-rate of large colorectal polyps in a blinded trial featuring tandem colonoscopy. Sixty-three lesions greater than or equal to 1 cm in size were discovered and none were missed. Confidence intervals of 95% are a miss-rate of 0 to 4.6%. We conclude that less than 5% of large colorectal polyps are missed during the index colonoscopic examination in a well-prepared colon.