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Purpose: Screening endoscopy has the potential to reduce colorectal cancer mortality. However, the efficacy of screening flexible sigmoidoscopy compared with colonoscopy strongly depends on the frequency of advanced proximal neoplasms without an index polyp in the rectosigmoid. We have therefore determined this frequency in our endoscopy population., Methods: Endoscopic and histologic data were analyzed from all patients on whom integral colonoscopy was performed between 1980 and 1995. Advanced neoplasia was defined as cancer or adenomas >10 mm in diameter, adenomas with a villous component, or severe dysplasia. Patients with polyposis syndrome or inflammatory bowel disease were excluded., Results: Colonoscopy was performed on 11,760 patients. 2,272 (19.3 percent) had at least one colorectal neoplasm, of which 39 percent had the neoplasm above the rectosigmoid. Twenty-two percent of all patients with neoplasia had no index polyp in the rectosigmoid and 16 percent of these had no index polyp, but at least one advanced proximal neoplasm., Conclusions: Although 39 percent of patients had neoplasms above the rectosigmoid, only 16 percent had an advanced proximal neoplasm without an index polyp in the rectosigmoid. This gives a figure on which to base the evaluation of screening sigmoidoscopy programs against those of screening colonoscopy.