We examined the relationships between iron deficiency anemia, immunochemical fecal occult blood test results, and colorectal cancer. Samples were collected from 17,664 asymptomatic individuals for an immunochemical occult blood test, and colonoscopy was carried out during medical check-up. The positivity rate on the occult blood test and detection rate for colorectal cancer were compared in those with and those without iron deficiency anemia. In addition, the detection rate for colorectal cancer was determined in four groups stratified by occult blood test results and the presence of iron deficiency anemia. The occult blood test was positive in 18.4% of those with and in 3.8% of those without iron deficiency anemia (P < 0.001), and colorectal cancer was detected in 2.7% of those with and in 0.4% of those without iron deficiency anemia (P < 0.05). The highest rate of colorectal cancer was found in subjects with both iron deficiency anemia and positive occult blood test result. These findings indicate positive associations between iron deficiency anemia and immunochemical occult blood test results, and between iron deficiency anemia and colorectal cancer. Colonoscopy is thus necessary particularly in cases of positive immunochemical occult blood test result and iron deficiency anemia.