A prospective colonoscopic study of 642 consecutive patients demonstrated that 156 (66%) mass lesions including 131 polyps and 25 cancers were located within 60 cm of the anus and 82 (34%) mass lesions including 68 polyps and 14 cancers were located beyond 60 cm, the working length of the rigid sigmoidoscope. One hundred twenty-one (61%) polyps and 14 (36%) cancers were located above 25 cm but less than 60 cm from the anus, the working length of the flexible sigmoidoscope. Sixty-eight (34%) polyps and 14 (36%) cancers were located beyond the reach of the flexible sigmoidoscope. Although the flexible sigmoidoscope detects more lesions than the rigid sigmoidoscope, 50% of colon cancers located beyond detection by the rigid sigmoidoscope would not have been detected with the flexible sigmoidoscope. Of all mass lesions located beyond 25 cm, 38% would not have been detected with the use of the flexible sigmoidoscope. It appears that total colonoscopy rather than flexible sigmoidoscopy must play an increasingly prominent role in the evaluation of patients with suspected colonic mass lesions.