Colorectal polyps are described in five patients with Cowden's disease. A brother-sister kindred and two unrelated patients had multiple colonic polyps 0.1-0.4 cm in diameter. Microscopical examination of seven polyps removed from these four patients showed distinctive lesions which were probably hamartomatous, and characterized by disorganization and proliferation of the muscularis mucosae with minimally abnormal overlying mucosa. The fifth patient had a solitary 2-cm epitheloid leiomyoma resected from the rectum. Review of all cases of Cowden's disease in the English literature suggests that gastrointestinal polyposis may be a frequent finding if it is specifically searched for. It does not appear that the previously reported association of Cowden's disease with gastrointestinal cancer is valid. The discovery of multiple colonic polyps 0.1-0.4 cm is diameter with a rectosigmoid distribution and a hamartomatous microscopical appearance is characteristic of Cowden's disease. Cowden's disease should be included in the differential diagnosis of gastrointestinal polyposis states.