We present the case of a woman with a giant endometrial polyp of uncommon dimension who was receiving adjuvant tamoxifen for breast cancer. In her gynecologic examination, she had a mass measuring 3 x 4 x 4 cm protruding from the cervical os. The mass was extirpated under general anesthesia. The mass originated from the endometrial cavity. The endometrial polyp measured 10 x 6 x 3 cm macroscopically and was found to be benign under microscopic examination. We conclude that physicians should be aware of the confounding effects of tamoxifen on the histological and ultrasonographic appearance of the endometrium.