Background: Noncardiac surgery is associated with significant cardiovascular mortality, morbidity, and cost. Small trials of β-blockers suggest that they may prevent cardiovascular events in patients undergoing noncardiac surgery, but trial results are inconclusive. We have initiated the POISE trial to definitively establish the effects of β-blocker therapy in patients undergoing noncardiac surgery. Methods: The POISE trial is a blinded, randomized, and controlled trial of controlled-release metoprolol versus placebo in 10000 patients at risk for a perioperative cardiovascular event who are undergoing noncardiac surgery. Patients will receive the study drug 2 to 4 hours before surgery and subsequently for 30 days. The primary outcome is a composite of cardiovascular death, nonfatal myocardial infarction, and nonfatal cardiac arrest at 30 days. Patients will also be followed for events at 1 year. Results: To date, the POISE trial has recruited >6300 patients in 182 centers in 21 countries. Currently, the patients' mean age is 69 years; 63% are males, 43% have a history of coronary artery disease, 43% have a history of peripheral arterial disease, and 30% have diabetes. Most participants have undergone vascular (42%), intraabdominal (23%), or orthopedic (19%) surgery. Conclusions: The POISE trial is a large international trial that will provide a reliable assessment of the effects of β-blocker therapy in patients undergoing noncardiac surgery.