The purpose of this study is to determine if patients with adult-onset diabetes mellitus having the surgical procedure called carotid endarterectomy benefit from increasing blood flow to the brain with a "shunt". A "shunt" is a tube inserted below and above the surgical area at the time when the carotid artery is clamped to allow the surgeon to work in a bloodless field and to supplement blood flow to the brain. Benefit is measured after surgery by comparing performance on a battery of cognitive tests, which test your ability to perform pencil-paper, memory and finger dexterity tests, to performance before surgery.