We aimed to estimate the proportions of colorectal cancer cases that might be prevented by sigmoidoscopy compared with colonoscopy among women and men. In a population-based case control study conducted in Germany, 540 cases with a first diagnosis of primary colorectal cancer and 614 controls matched for age, sex, and county of residence were recruited. A detailed lifetime history of endoscopic examinations of the large bowel was obtained by standardized personal interviews, validated by medical records, and compared between cases and controls, paying particular attention to location of colorectal cancer and sex differences. Overall, 39%, 77%, and 64% of proximal, distal, and total colorectal cancer cases were estimated to be preventable by colonoscopy. The estimated proportion of total colorectal cancer cases preventable by sigmoidoscopy was 45% among both women and men, assuming that sigmoidoscopy reaches the junction of the descending and sigmoid colon only and findings of distal polyps are not followed by colonoscopy. Assuming that sigmoidoscopy reaches the splenic flexure and colonoscopy is done after detection of distal polyps, estimated proportions of total colorectal cancer preventable by sigmoidoscopy increase to 50% and 55% (73% and 91% of total colorectal cancer preventable by primary colonoscopy) among women and men, respectively. We conclude that colonoscopy provides strong protection against colorectal cancer among both women and men. The proportion of this protection achieved by sigmoidoscopy with follow-up colonoscopy in case of distal polyps may be larger than anticipated. Among men, this regimen may be almost as effective as colonoscopy, at least at previous performance levels of colonoscopy.