A 47-year-old women referred to our hospital with sudden left lower abdominal pain and state of shock in April 2006. Computed tomographic (CT) scan revealed a retroperitoneal hematoma and we suspected a renal tumor or angio myolipoma but enhanced CT scan show bleeding from a left renal artery. We perfomed left renal artery angiography and admitted a arteryal stenosis. Finally we diagnosed renal artery aneurysm caused by fibromuscular dysplasia. We performed left radical nephrectomy on the same day. We report the details of this case.