It has become promising to establish a method for mass screening of colorectal cancer through the development of immunological fecal occult blood test. It has been shown that such tests have higher specificity and sensitivity compared to conventional chemical tests not only in cancer subjects and normal controls but in population screening trials. Further study is required to decide the optimal system within the framework of the screening such as the number of specimens, target age, and screening interval, etc. The efficacy of fecal occult blood screening remains to be shown in spite of the several randomized controlled trials completed or on-going. However, a case control study has strongly suggested that such screening would reduce the mortality from colorectal cancer.