In order to search for the histogenesis and progression of colorectal cancer, the background mucosas of 18 patients with a single colorectal cancer, apart from familial adenomatosis coli, were studied by a step sectioning method. Three early minute cancers (3-5 mm in diameter) were detected in the apparently normal mucosa, and two cancers in the adenoma (focal cancer in adenomatous polyp). The three early cancers had no evidence of preexisting adenoma, so they were considered to be de novo cancers. In form they were depressed, flat, and slightly elevated. In humans, de novo cancer has an important significance in histogenesis and treatment of colorectal cancer. Clinically, it is also important to recognize the early phase of colorectal cancer in flat type as well as polypoid type.