Background: Malignant transformation of endometriosis is very rare and implies considerable diagnostic difficulties. Depending on the implantation site, it may present various symptoms. The ovary is the most common site, followed by the pelvic peritoneum and the rectovaginal septum. The following report describes a case of endometrioid adenocarcinoma in endometriosis of the ureter. Case Report: A 57-year-old patient presented herself in our clinic with a weight loss of 8 kg in 3 months, abdominal pain and hematuria. 13 years before, an abdominal hysterectomy and right-sided adnectomy because of endometriosis and uterus myomatosus had been performed, followed by hormone replacement therapy with oral estrogens. Computer tomography and abdominal sonography showed a hydronephrotic right kidney. Scintigraphy showed loss of function of the right kidney. Surgery was performed and the tumor, including the right kidney and the right ureter, was removed. Histopathological examination showed malignant transformation of ureteral endometriosis. Radiation therapy of the pelvic region and the draining lymphatic system was applicated. 18 months after operation and adjuvant radiotherapy, the patient was free of symptoms, and there was no evidence of disease. Conclusion: After diagnosis of endometriosis and especially after longterm hormone replacement therapy with estrogens alone, in patients with unclear abdominal symptoms, the possibility of malignant transformation of endometriosis should be taken into consideration.