Fifty-one early carcinomas not more than 1 cm in size (small early carcinoma) and 88 flat elevations collected from polypectomy and operative specimens were analyzed colonoscopically and histologically. Flat elevation is defined here as a flat elevated lesion found on endoscopy and it is either adenoma or early carcinoma. On histology its height is almost twice as thick as surrounding normal mucosa. In small early carcinoma broad based type was the commonest and all of small flat type carcinomas were confined to the mucosa. Among 88 flat elevations there were 16 mucosal and 4 submucosal carcinomas making 22.7% of malignancy rate. The malignancy rate of flat elevation increases with increasing size from 9.1% to 55.6%. Most of flat elevations which were demonstrated as slightly elevated reddish lesions with occasional central depression were benign adenomas under 1 cm in diameter. No significant correlations between the grade of atypia and the presence or absence of central depression were noticed. All flat carcinomas but one contained adenomatous components suggesting adenoma-carcinoma sequence. Flat elevations were more frequently situated distally but flat carcinomas, classified as IIa, showed even distribution. Although natural history of flat elevation is not clear at present, it is most likely to be an origin of IIa or IIa + IIc type of carcinoma and even so-called "de novo" carcinoma may arise from preexisting flat adenoma. It is suggested that flat elevation (adenoma and carcinoma) can be another origin of colonic carcinoma.