The involvement of the retroperitoneum by endometriosis occurs only sporadically. In the present report, emphasis will be given to the magnetic resonance imaging findings which raised the diagnostic suspicion of periureteral lesion in a patient undergoing tamoxifen therapy, presenting with left upper quadrant pain and hydronephrosis. Histopathological findings obtained by means of computed tomography-guided percutaneous biopsy revealed the diagnosis of periureteral endometriosis.