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To the Editor: Dr Frazier and colleagues¹ estimate that providing patients aged 50 years and older at average risk for colorectal cancer with screening colonoscopy every 10 years will reduce colorectal cancer mortality by 64%. These results are similar to those reported by Sonnenberg et al.² The standard evidence given in support of this dramatic mortality benefit, which is substantially greater than has been demonstrated for any screening test, is the National Polyp Study.³ This study was not a randomized trial of the effect of colonoscopy on colorectal cancer mortality but a cohort study of selected patients undergoing colonoscopy. Because the National Polyp Study was not a randomized trial, the comparability of the case and control groups (3 historical cohorts) is open to question. Furthermore, the study's end point was the incidence of colorectal cancer, not mortality. This would not matter if every incident cancer resulted in death, but that . . . [Full Text of this Article]