We report a case of lumbosacral plexopathy caused by the rupture of a common iliac artery aneurysm. The patient presented with sciatic type symptoms of lower back pain radiating to his left leg with associated numbness and weakness in the L4-S1 distribution. He also had reduced anorectal tone. A CT scan showed a large haematoma in the left side of the pelvis from a ruptured 8 cm common iliac artery aneurysm. Sciatica is commonly due to a prolapsed intervertebral disc, although spinal canal stenosis, spondylolisthesis, piriformis syndrome and spinal tumours and other causes need to be considered. This case serves to increase the awareness of the possibility of another uncommon cause, especially when additional atypical neurological symptoms exist.