To determine the occurrence of synchronous large bowel polyps located proximal to the sigmoid, in persons undergoing screening flexible sigmoidoscopy, we examined those who had diminutive polyps (≤ 0.5 cm) as the only finding in the distal colon by further colonoscopy. One hundred one asymptomatic persons (mean age 61 ± 13 years) had 143 diminutive polyps, a single polyp was found in 76%, and 64% of all polyps were located in the rectum. Thirty (21%) were hyperplastic and 86 (60%) were neoplastic, including 14 with moderate and one with severe dysplasia. The others were inflammatory (five) or unclassified (hot biopsy changes or normal mucosa, 14 polyps), and eight were lost before processing. Colonoscopy revealed that 16 (16%) of the 101 patients had 21 additional polyps proximally, mostly < 1 cm in diameter. These included one hyperplastic and 18 neoplastic polyps, and two specimens showed hot biopsy changes. Age, histological type, number or location of the index diminutive polyps, were not associated with proximal lesions. We question whether immediate colonoscopy is justified in asymptomatic patients with only diminutive polyps at flexible sigmoidoscopy.