Isolated internal iliac aneurysms are rare. Unless rupture occurs, they usually remain asymptomatic. In this paper, a patient with a ruptured internal iliac aneurysm that resulted in chronic stage hematoma causing lumbosacral plexopathy and erosion of the pelvic bony structures is presented, along with magnetic resonance imaging (MRI) and computed tomography (CT) findings. To expedite the diagnosis of extraspinal radicular pain, one should pay attention to the extraspinal structures involved in lumbar CT or MRI examinations. If necessary, further investigation can be made with pelvic CT or MRI.