Carcinomas, adenomas and nonneoplastic polyps of the large intestine were examined histopathologically for evidence of recent and old hemorrhage. Three percent of the carcinomas lacked evidence of hemorrhage, as did 7% of the adenomas. Fifty percent of hyperplastic polyps were negative for hemorrhage, but 23% had considerable hemorrhage or hemosiderin. These findings may have a bearing on the results in fecal occult blood testing for the detection of cancer. Carcinomas bleed mainly from erosions on their luminal surface, but adenomas rarely show this feature. Twisting and bending of polyps, leading to fracture of fronds, and vascular obstruction, with passive congestion and stromal hemorrhage, appear to be the main sources of bleeding in polyps. Adenomas on the left side of the large intestine showed greater evidence of bleeding than did lesions on the right, but carcinomas did not show this relationship. Factors correlated with more severe hemorrhage were size, pedunculation, and villous growth of adenomas; for carcinomas, they were size, lack of mucin content, and lower differentiation. The age and sex of the patient and the extent of spread of carcinomas did not correlate with hemorrhage.