The primary objective of the study is to determine whether enoxaparin compared to unfractionated heparin will reduce the composite endpoint of all-cause mortality and non-fatal myocardial re-infarction within 30 days after randomization in patients with acute ST-segment elevation myocardial infarction who are eligible to receive fibrinolytic therapy
INCLUSION CRITERIA: Patients with ST-segment elevation acute myocardial infarction meeting all of the following criteria: * Male or non-pregnant female greater than or equal to 18 years of age (depending on local regulations, minimal age can vary between 18 and 21 years) * Onset of prolonged (greater than or equal to 20 min) ischemic symptoms at rest less than or equal to 6 hours prior to randomization * ST-segment elevation of 0.1 mV in 2 or more limb leads, or 0.2 mV in two (2) or more contiguous precordial leads, or left bundle-branch block * Planned reperfusion therapy with streptokinase, tenecteplase, alteplase or reteplase * Written informed consent will be obtained EXCLUSION CRITERIA: Cardiovascular * Evidence of cardiogenic shock at randomization * Acute pericarditis * History or symptoms suggestive of aortic dissection * MI precipitated by obvious provoking factors such as arrhythmia, infection, severe anemia, hyperthyroidism, cocaine, or amphetamine Hemorrhagic Risk * Any minor head trauma or any other trauma occurring after the index acute myocardial infarction * Active or recent (\< 3 months) bleeding including gastrointestinal bleeding, known presence of occult blood in the stool, or gross hematuria. * Any history of bleeding diathesis, coagulopathy, platelet disorder, or thrombocytopenia * Any single reliable recording of systolic blood pressure \>180 mm Hg and/or diastolic blood pressure \>110 mm Hg prior to randomization * Any history of stroke or transient ischemic attack; any history of hemorrhagic cerebrovascular disease * Any known structural damage or other pathologic process involving the central nervous system * Any head trauma within 6 months prior to randomization * Major surgery (including CABG), any ophthalmologic surgery, or non-cutaneous biopsy, or substantial trauma within 3 months prior to randomization * Traumatic or prolonged cardiopulmonary resuscitation (\> 2 minutes) within 2 weeks prior to randomization * Puncture of a non-compressible vessel (artery or vein) within the 24 hours prior to randomization * Acute peptic ulcer disease within 3 months prior to randomization Prior or Concomitant Pharmacologic Therapy * Administration of abciximab (ReoPro), within the previous 7 days or eptifibatide (Integrilin), or tirofiban (Aggrastat) within the previous 24 hours prior to randomization * Current therapy with oral anticoagulants, or an International Normalized Ratio of \>1.5 * Administration of a low molecular weight heparin within 8 hours prior to randomization. * Known hypersensitivity to low molecular weight heparins, unfractionated heparin or heparin-like products; allergy to pork or pork products * Known hypersensitivity and/or contra-indication(s) to fibrinolytic drugs (streptokinase, tenecteplase, alteplase and reteplase) General * Known platelet count \<100,000 cells/microL or history of heparin-induced thrombocytopenia * Known clinically significant anemia (Hemoglobin \<10 g/dL which is \< 6.2 mmol/L) * Known renal insufficiency with serum creatinine \>220 mmol/L (2.5 mg/dL) for men and \>175 mmol/L (2.0 mg/dL) for women when assessed prior to baseline examination. * Advanced neoplastic or other life-threatening disease with a life expectancy of \<12 months * Pregnancy or parturition within the last 90 days or currently breast feeding * Women of childbearing potential except if post-menopausal, surgically sterile or using accepted method(s) of birth control or having a negative pregnancy test. * Treatment with other investigational agents in the last 30 days before study entry or previous enrollment in ExTRACT-TIMI 25 * History of drug or alcohol abuse * Mental condition rendering the patient unable to understand the nature, scope, and possible consequences of the study * Any patient unlikely to comply with protocol, e.g., uncooperative attitude, inability to return for follow-up visits, and who are unlikely to complete the study