Diminutive polyps are frequent findings on screening flexible sigmoidoscopy. To determine the significance of distal diminutive polyps, we conducted a prospective study of 162 asymptomatic, average-risk subjects who were 50 years of age or older. Subjects were divided into four groups: 42 control subjects with no polyps in the rectosigmoid, 66 subjects with at least one diminutive adenoma in the rectosigmoid, 12 subjects with a mixed hyperplastic-adenomatous polyp in the rectosigmoid, and 42 subjects with only hyperplastic polyps in the rectosigmoid. Total colonoscopy was performed on all subjects. The prevalence of proximal adenomas was 42% in the adenoma group, 25% in the mixed group, 14% in the hyperplastic group and 12% in the control group. The prevalence of proximal adenomas was significantly higher (p = 0.006) in the adenoma group as compared with the control and hyperplastic groups. Increasing age was associated with an increased prevalence of proximal adenomas. Nearly two thirds of those over 65 years of age with distal diminutive adenomas had proximal colonic neoplasms. These results indicate that diminutive rectosigmoid adenomas are good markers for proximal neoplasms. Rectosigmoid hyperplastic polyps are not associated with an increased prevalence of proximal neoplasms. Total colonoscopy is not indicated if hyperplastic polyps are the only finding on flexible sigmoidoscopy.