Background and Study Aims: The recognized pathway for colorectal malignancies is the adenoma-carcinoma sequence. It is estimated that up to two-thirds of colorectal carcinomas arise from adenomatous polyps. In recent years, Japanese workers have suggested that early colorectal malignancies may arise as “flat” or “depressed” rather than as polypoid lesions. Such flat or depressed adenomas and adenocarcinomas have not been widely recognized in the West. A prospective study was carried out to search for flat and depressed adenomas in a British population, using Japanese colonoscopic techniques. Patients and Methods: In this prospective study at a British centre, 210 consecutive patients attending for routine colonoscopy were examined for flat or depressed lesions. The examinations were carried out using Japanese techniques by an experienced Japanese endoscopist (T.F.). Results: Overall, 68 adenomas were found, of which 40 (59 %) were polypoid, 26 (38 %) were flat, and two (3 %) appeared depressed. The majority of the adenomas contained areas of mild or moderate dysplasia, but four were severely dysplastic. Two of these were large polypoid tubulovillous adenomas, the third was a 7-mm protruding polyp, and the fourth was a depressed adenoma 6 mm in diameter. Three Dukes' stage A adenocarcinomas were also found. Of these, one was a 20-mm polyp, one a 15-mm flat elevation of the mucosa with a central depression, and the third a 6-mm depression of the mucosa. Finally, four Dukes' B or more advanced adenocarcinomas were found. Conclusions: The polyp-carcinoma hypothesis prompts colonoscopists to search only for polypoid lesions when screening for malignancy. A significant proportion of early colorectal neoplasms may therefore be missed. European colonoscopists require training in the recognition of flat elevated and depressed lesions in order to detect colorectal malignancies in their early stages.