We report the results of a free, television-advertised mass screening program for colorectal cancer using stool guaiac kits. A total of 57,000 test kits were picked up and 29,619 (53%) were returned; 3.9% (1165) of the tests were positive. Ninety-three percent of persons with a positive screen sought medical evaluation after screening. Detailed follow-up was available on 744 persons. Fifty-eight persons had large-bowel carcinomas diagnosed, 80% of which were localized. One hundred sixty persons had adenomatous polyps removed. Forty percent of cancers and 58% of polyps were detected in persons with only one or two positive test slides out of a total of six. In 33% of persons with a positive screen, the diagnostic workup consisted of a repeated stool guaiac test and/or sigmoidoscopy only. A major drawback to improving the results of mass screening programs for colorectal cancer is the limited gastrointestinal workup conducted by physicians in many persons with a positive fecal occult blood test.