To characterize the endoscopic features of minute non-polypoid neoplastic lesions of the colon, we investigated the endoscopic, macroscopic, and histologic findings in 34 lesions detected by colonoscopy, which were smaller than 5 mm and were endoscopically recognized as flat or depressed lesions. The lesions were divided into two groups according to their endoscopic features; 8 lesions were completely flat and the remaining 26 lesions were flat-topped elevations with a central depression. Macroscopic examinations revealed that the center of the endoscopically flat lesion was slightly elevated, whereas the center of the flat-topped elevation was characterized by a central depression. All of the lesions were histologically diagnosed as tubular adenomas. The flat lesions were composed of flatly elevated adenomatous glands, while the flat-topped elevations with a depression were characterized by surrounding hyperplasia arising from normal glands. The whole thickness of the mucosa was replaced by adenomatous glands in the lesions with an obvious central depression, while the adenomas tended to spread superficially in the lesions with a shallow depression. These findings suggest that pre-cancerous lesions other than small polyps do exist in the colon and that colonoscopic examination provides some clue toward a prediction of the histologic architecture of the lesions.