As the first screening for colorectal neoplasms, a total of 2243 examinations by fiberoptic sigmoidoscopy (FS) were performed on 1573 asymptomatic patients. One hundred seventy neoplasms were found in 134 patients (6 percent of total examinations), including nine adenocarcinomas (0.4 percent). A flat, intramucosal cancer and a small, 5-mm cancer detected by FS could not be demonstrated by barium enema examination. On further investigation, additional adenomas were identified in the proximal colon in 22 percent of patients in whom neoplasms had first been detected by FS and in only 4.4 percent of those in whom FS findings were negative but other criteria were positive. It is concluded that FS is the procedure of choice for the first screening of asymptomatic patients, not only because it enables detection of flat and small lesions within its observation range, but also because it could lead to detection of proximal neoplasms.