This randomized, placebo controlled study will test the hypothesis that perioperative estrogen replacement in postmenopausal women reduces the risk for neurologic injury after CABG surgery. Three hundred thirty-four women undergoing CABG surgery will be prospectively randomized to receive either 17 beta-estradiol or placebo in a double-blind fashion beginning the day before surgery and continuing for 5 days after surgery. Patients will be assessed for neurocognitive dysfunction, which is the most common manifestation of neurologic injury from cardiac surgery. Neurocognitive testing will be performed 1 to 2 days before surgery, 4 to 6 weeks postoperatively, and 6 months after surgery. The primary endpoint will be neurocognitive function 4 to 6 weeks after surgery for women who received 17 beta- estradiol compared with placebo perioperatively. The trial will also evaluate the importance of postoperative cognitive decline on measures of cognitive function and quality of life 6 months after surgery, and whether perioperative 17 beta-estradiol treatment improves these outcomes.