In a patient on oral anticogulation with sudden onset of loin pain, the possibility of spontaneous bleeding in the retroperitoneum must be considered in the differential diagnosis of renal colic. This rare pathology can be life-threatening, and rapid diagnosis with ultrasound and computerized tomography of the abdomen must be made. If the patient is hemodynamically stable, a conservative approach is justified. Otherwise, emergency nephrectomy may be needed. As patients become older and indications for anticoagulant therapy become more common, we will probably have to face more of these rare bleeding complications in the future. We report such a case.