The clinicopathologic and endoscopic features of 15 patients with small flat cancer of the rectum were investigated. Whereas 4 patients had hematochezia, the remaining 11 patients were asymptomatic, and stool positive for occult blood was the only remarkable clinical feature in 8 of them. Endoscopic features were slight elevation with central depression in 8 lesions, flat-topped elevation in 4, and shallow depression with irregular margin in 3. The surface of the tumor was often faint red in color and frequently characterized by mucosal friability. Five flat rectal cancers were missed during initial endoscopy, and they were found by repeated endoscopic examination. Three flat cancers were not identified in the distal rectum until retroflexed colonoscopic observation was performed. Although all tumors were smaller than 2 cm in diameter and 6 of them were under 1 cm, 9 lesions had deeply invaded the submucosal layer and 7 tumors showed lymphovascular permeation. Two lesions of 1 cm or greater had metastasized to perirectal lymph nodes. These results suggest that careful observation during endoscopy is necessary to detect flat rectal cancers, and a U-turn to examine the anorectal junction should be routinely done in the appropriate age group.