Persistent sciatic artery is an embryonic blood vessel that continues to feed the lower extremity after fulfilling an important role in lower limb development during early gestation. It is so rare that only 20 cases have been reported in Japan. This paper describes a case of acute occlusion of a persistent sciatic artey. A 78-year-old woman was admitted to hospital because of sudden onset of severe pain in her left leg. Angiography showed bilateral persistant sciatic arteries (complete type) with occlusion of the left artery and a small aneurysm on the right side. Left femoro-popliteal bypass was performed and postoperative angiography showed that the graft was patent.