This proposed study will determine whether early treatment with intravenous magnesium will reduce short-term mortality of high risk patients with suspected acute myocardial infarction (presenting with ST elevation on the ECG). Treatment will be administered sufficiently early to reduce reperfusion injury. The primary endpoint will be all-cause 30 days mortality. To define the mechanisms of benefit from magnesium treatment and assess the tolerableness of magnesium, secondary endpoint will include the development of cardiogenic shock, ventricular fibrillation, congestive heart failure, and conduction delay that requires the placement of a temporary pacemaker. To accomplish this objective, the contractor will conduct a multicenter, placebo-controlled, double blind trial that will randomize approximately 10,400 patients with suspected acute myocardial infarction. Patients who do not receive thrombolytic therapy or primary angioplasty (PTCA) will be randomized if they can begin their magnesium infusion within 6 hours of the onset of symptoms. Patients greater than or equal to 65 years of age undergoing reperfusion therapy (either with thrombolytics or PTCA) will be eligible provided magnesium can be administered before or at the same time as the reperfusion therapy. Patients will be stratified at randomization based on whether or not they receive reperfusion. Details of the study design, including entry criteria, randomization procedures, and analyses which will be incorporated in the protocol will be developed by a Steering Committee that will include the New England Research Institutes.