The main mechanism responsible for the sanitary impact of ischemic heart disease is cardiomyocyte cell death associated to acute coronary syndrome with ST segment elevation (ACSST). In most of these patients, performing PTCA or thrombolysis as soon as possible does not prevent the occurrence of myocardial necrosis involving a substantial portion of the area at risk. Intracoronary adenosine (ADO) at the time of reperfusion limits infarct size in animals, and preliminary clinical studies indicate that may be also protective in patients with ACSST receiving early reperfusion therapy. OBJECTIVE: to evaluate the safety and efficacy of a brief intracoronary infusion of ADO applied at the time of reperfusion to limit infarct size and LV remodelling in patients with ACSST submitted to primary ACTP. DESIGN: Multicentric, prospective, randomised, parallel, placebo-controlled double-blind study. PATIENTS: 200 patients older than 18 with ACSST and without prior myocardial infarction receiving primary PTCA within 6 hours after symptom onset. PROTOCOL: Intracoronary ADO (4mg) or placebo (saline) infusion distal to the culprit lesion immediately before stent deployment, NMR between 6 and 14 days and after 6 months. END-POINTS: Major: infarct size as measured by NMR, Secondary: changes in LV volumes and EF, and major cardiac events during the follow-up.