The prevalence of polyps and cancer in the proximal colon among patients who have polyps detected on sigmoidoscopy was determined in a large rural referral hospital in north central Pennsylvania. Eleven thousand one hundred sixty patients underwent sigmoidoscopy between 1991 and 1997. Polyps were detected in 709 patients. Five hundred twenty-three patients who had a polyp at sigmoidoscopy and full colonoscopy completed within one year were included in this study. 120 patients (23%) had a proximal polyp detected at colonoscopy. The prevalence of proximal polyps and histologically advanced polyps was related to the size, number, and histology of the distal index polyp found at sigmoidoscopy. However, the absolute difference in prevalence of proximal polyps stratified by dings at sigmoidoscopy was small. A total of 5 adenocarcinomas were detected in the proximal colon. All proximal cancers detected at colonoscopy occurred in patients with a distal polyp less than 10 mm. Our data emphasize the importance of colonoscopy in all patients with a polyp detected at sigmoidoscopy independent of its size and histology.