The aim of this study was to document a case of tamoxifen-associated extensive pelvic endometriosis and attract the attention to this side effect of tamoxifen use in the postmenopausal patient. A 74-year-old woman with a history of breast carcinoma who received tamoxifen therapy for 2 years was admitted with uterine bleeding. Hysteroscopic polypectomy revealed a hyperplastic polyp. Extensive pelvic endometriosis was detected at the operation and due to dense adhesions, subtotal hysterectomy and bilateral salpingo-oophorectomy were performed. The patient continued to use tamoxifen. A supracervical pelvic mass was detected 14 months later. The cervix, rectum, and the accompanying mass were resected. Histopathologic examination revealed endocervical adenocarcinoma and endometriosis involving cervix uteri and the rectal muscular wall. The patient had two normal cervical smears within the last 3 years and no abnormal appearance was detected within the cervical canal in the hysteroscopic examination. As cervical cancer occurred in a short period, it might be speculated that tamoxifen might have stimulated the proliferative and mitotic activity of cervical endometrial tissue which has progressed into invasive cancer in time.