Performing screening flexible sigmoidoscopy using colonoscopes: experience in 500 subjects. There is still controversy regarding the optimal length of flexible sigmoidoscopes. We performed screening distal colon examinations using 168-cm colonoscopes in 500 asymptomatic subjects who were unsedated and had sigmoidoscopy cleansing preparation. The mean depth of penetration was 66 cm and was similar in persons in whom the examination was discontinued because of poor preparation versus those with discomfort. Polyps were detected in 87 patients, but only 5 subjects had polyps detected above 60 cm. We conclude that in a group of unsedated subjects scheduled for flexible sigmoidoscopy after a sigmoidoscopy prep, the use of instruments longer than 60 cm gives very little additional yield. 