OBJECTIVE To describe how a hydronephrosis can lead to a difficult differential diagnosis between endometriosis and retroperitoneal fibrosis. METHODS Case report. METHODS Department of Obstetrics and Gynecology, Sacro Cuore Don Calabria General Hospital, Negrar, Verona, Italy. METHODS The history of a 34-year-old woman revealed the appearance of hydroureteronephrosis on the right side at the 35th week of pregnancy. She had an magnetic resonance imaging scan and was diagnosed with a spread retroperitoneal fibrosis. After 2 months, the patient reported the occurrence of pelvic pain, dyspareunia and dysmenorrhea. She was treated with corticosteroids and tamoxifen with no results. METHODS Laparoscopic surgery. A complete retroperitoneal extirpation was done of an endometriotic nodule of the right broad ligament, near the right ureter (without stenosis). METHODS Reduction of pelvic pain. RESULTS She noticed an important decrease of pain. CONCLUSIONS The cause of hydronephrosis could be a physiologic hydroureteronephrosis, which is the most common cause of dilatation of the urinary tract in pregnancy. The pain symptoms of the patients seemed to be linked to endometriosis and not to retroperitoneal fibrosis. Magnetic resonance imaging sometimes does not enable a correct diagnosis between these two pathologies. Fertile women with suspected fibrosis should undergo a diagnostic laparoscopy by an expert surgeon in retroperitoneal surgery.