Multiple uterine leiomyomas are present in a large population of women and may cause several uncommon clinical symptoms, including disseminated vein thrombosis and hydronephrosis. We report a case of giant uterine leiomyomas causing bilateral hydronephrosis coexisting with endometrial cancer (EC) deriving from a uterine polyp. A 50-year-old woman was admitted due to bilateral hydronephrosis caused by monstrous abdominal tumor to the IInd Department of Gynecology, Lublin Medical University, Lublin, Poland. A bilateral double-J catheter was inserted. Pelvic examination revealed a huge, rough tumor, originating from the uterus. Total abdominal hysterectomy with bilateral salpingo-oophorectomy was performed, and a giant uterus weighing 15.2 kg and measuring 35 x 29 x 18 cm was removed. Histopathological examination revealed multiple uterine leiomyomas with calcification and partial necrosis, and well-differentiated (G1), endometrioid-type EC (Stage IA) concomitant with atypical endometrial hyperplasia, deriving from a uterine polyp. The postoperative recovery was without complications, and the patient was discharged on postoperative day 10. In conclusion, giant uterine leiomyoma may incidentally compress the urinary tract organs, causing hydronephrosis.