OBJECTIVE This study was designed to investigate the validity of a single immunochemical fecal occult blood test (FOBT) for detection of colorectal neoplasia. METHODS A total of 3,794 average-risk screenees and 304 colorectal cancer patients admitted to the National Cancer Center, Korea, between May 2001 and November 2002, were studied prospectively. All screenees and admitted patients underwent FOBT and total colonoscopic examinations. Stools were self-collected, and examined using an immunochemical fecal occult blood test (OC-hemodia, Eiken Chemical Co. Tokyo, Japan) and an OC-sensor analyzer(R) (Eiken Chemical Co. Tokyo, Japan). RESULTS Of the 3,794 asymptomatic screenees, the colonoscopy identified colorectal adenomas and cancers in 613 (16.2%) and 12 (0.3%) subjects, respectively. The sensitivities of a single immunochemical FOBT for detecting colorectal cancers and adenomas in screenees were 25.0 and 2.4%, respectively. The false positive rate of FOBT for colorectal cancer in screenees was 1.19%. For the total 316 colorectal cancer cases (including 12 cases from screenees), the FOBT sensitivities according to the T-stage were 38.5, 75.0%, 78.9 and 79.2% for T1, 2, 3 and 4 cancers, respectively. The sensitivities according to the Dukes stages A, B and C were 63.4, 79.3 and 78.6%, respectively. CONCLUSIONS The sensitivities of a single immunochemical FOBT for detecting colorectal cancers and adenomas in screenees were 25.0 and 2.4%, respectively. The sensitivities of FOBT were about 80% for Dukes B or C colorectal cancers and 63.4% for Dukes A.