The histologic features of an endometrial polyp include irregular, often dilated glands, thick-walled blood vessels, and a fibrotic stroma. Such polyps may be responsive to some chemotherapeutic drugs that can exert hormonal effects. We report on endometrial polyps detected in three postmenopausal patients who were receiving tamoxifen for treatment of metastatic breast carcinoma. The clinical presentation in all cases was vaginal bleeding and all had documented uterine enlargement suggestive of an intrauterine malignancy. The polyps were large, measuring up to 9 cm in largest diameter. On histologic examination each polyp had extensive cystic glandular hyperplasia. In one case foci of atypical epithelial proliferation and predecidualization were noted. The atypical proliferation suggested a borderline neoplastic process and was strongly positive for carcinoembryonic antigen. These findings underscore the marked proliferative changes that can be induced in endometrial polyps in postmenopausal women receiving hormonally active chemotherapeutic agents.