We investigated the endoscopic and histologic features of 74 nonpolypoid neoplasias of the colorectum measuring 15 mm or less in diameter. Colonoscopically, they were characterized by flat growth, being either slightly elevated with a central depression or else completely flat against the surrounding mucosa. Of the 54 lesions measuring 5 mm or less in diameter, 3(7%) were diagnosed as cancer, whereas the remaining 51 (93%) were diagnosed as adenomas with moderate epithelial atypia. The lesions measuring more than 5 mm were fewer in number (20), and they were more frequently given the histologic diagnosis of severe epithelial atypia (20%) and cancer (70%). Whereas the endoscopic features of the cases of adenoma with moderate atypia included both complete flatness and flat-topped elevation with a central depression, the lesions diagnosed as adenoma with severe atypia were endoscopically recognized as flat or slightly depressed lesions, and a deep central depression or prominent elevation was noted in the lesions diagnosed as cancer. These findings suggest that some minute nonpolypoid neoplasias may transform to nonpolypoid cancers and invade the submucosa, whereas others do not manifest rapid growth.