Reduced mortality from colorectal cancer may be achieved by screening with faecal occult blood testing. Screening for neoplasia in the rectum and sigmoid colon with flexible sigmoidoscopy is suggested to be more effective, particular among persons between 50 and 60 years of age. A cohort of 6367 persons 55-56 years of age were randomised to screening with rehydrated Hemoccult II tests (HII group) or with flexible videosigmoidoscopy directly (FS group). In the HII group 59% (1893/3183) attended, compared to 49% (1353/3184) in the FS group. Of the 1893 persons who attended in the HII group, 4% had a positive HII test and in 13% (10/78) of them a neoplasm > or = 1 cm in the rectum or sigmoid colon was diagnosed by endoscopy. The corresponding rate in the FS group was 2.3%. Overall the number of persons with a neoplasm > or = 1 cm diagnosed in the HII group was 10 and in the FS group 31. A subgroup in the flexible sigmoidoscopy group, who also performed rehydrated HII tests, showed a sensitivity of the HII test for neoplasia > or = 1 cm of 26% and a specificity of 95.6%. To find a neoplasm > or = 1 cm in the rectum or sigmoid colon, 44 examinations were needed when using flexible sigmoidoscopy directly and 7 examinations when only those with positive HII tests were examined. In mass screening for neoplasia in the rectum and sigmoid colon, the relatively low prevalence of colorectal neoplasia at 55-56 years of age makes primary selection with rehydrated Hemoccult testing an alternative to the resource-consuming endoscopy of all invited persons.