As the first step in the epidemiological evaluation of the effectiveness of mass screening for colorectal cancer, we compared clinicopathological features and survival rates of patients with cancer detected by mass screening (screened group) with those for patients treated in our outpatient clinic in the same period (outpatient group). The screened group consisted of 53 patients with colorectal cancer detected by 2-day or 3-day screening for fecal occult blood by guaiac slides. Their background factors were comparable to those of 120 patients of the outpatient group in regard to age, sex ratio, location of cancer, and histological type of cancer. In the screened group, 90% of patients had no complaints, and positive occult blood tests led to the detection of cancers. More than 60% of the patients had Dukes' A and B1 early cancers while only about 30% had advanced cancers. In the outpatient group, nearly 90% of patients were symptomatic, most commonly from rectal bleeding. Early-stage cancers made up only 20%, and large, advanced-stage cancers accounted for 80%. The 5-year survival rate of the screened group was 91.5%, being significantly higher than the 60% survival rate for the outpatient group. It is anticipated that mass screening for colorectal cancer by guaiac fecal occult blood testing will significantly reduce the mortality due to this neoplasm.