A prospective study was undertaken to investigate the feasibility and diagnostic yield of ileoscopy as an extension of total colonoscopy. The distal 15 to 40 cm of the terminal ileum were visualized in 400 of 555 consecutive patients submitted to total colonoscopy (72 percent). Intubation of the terminal ileum was not successful in 42 patients (8 percent), not tried in 63 (11 percent), and hampered by inadequate colonic cleansing in 50 (9 percent). We observed 13 patients with terminal ileitis and seven rare findings such as nonspecific ileal ulcer or non-Meckelian diverticulum, etc., adding to a total of 20 pathologic findings in 400 examinations (5 percent). In a further group of 98 patients, useful diagnostic information was gained by the demonstration of normal ileal findings, making ileoscopy clinically valuable in 118 of 400 examinations (29.5 percent) in this series. We suggest routine endoscopy of the terminal ileum in all patients with suspected or established inflammatory bowel disease and/or persistent diarrhea, lower gastrointestinal tract bleeding, or irritable bowel syndrome, in whom the diagnostic procedure includes colonoscopy. In patients with colorectal tumors or polyps, the diagnostic yield will be unrewarding.