Forty-two patients underwent flexible sigmoidoscopy in the radiology department. After maximal insertion of the sigmoidoscope, a supine abdominal film documented the position of the tip of the instrument. Correlation with barium enema in all patients determined the extent of visualization of the colon relative to the length of insertion of the sigmoidoscope. Insertion to 60 cm was achieved in 31 (74%), 40-59 cm in six (14%), 25-39 cm in three (7%), and under 25 cm in two (5%), with a mean depth of insertion of 54 cm. Full insertion examined the entire sigmoid colon in 14 (45%) of 31 patients. Insertion between 40-59 cm achieved complete sigmoid evaluation in only one (17%) of six patients. Thus, flexible sigmoidoscopy does not necessarily guarantee evaluation of the entire sigmoid colon.