OBJECTIVE To assess effect of intravenous recombinant tissue type plasminogen activator on size of infarct, left ventricular function, and survival in acute myocardial infarction. METHODS Double blind, randomised, placebo controlled prospective trial of patients with acute myocardial infarction within five hours after onset of symptoms. METHODS Twenty six referral centres participating in European cooperative study for recombinant tissue type plasminogen activator. METHODS Treatment group of 355 patients with acute myocardial infarction allocated to receive intravenous recombinant plasminogen activator. Controls comprised 366 similar patients allocated to receive placebo. METHODS All patients were given aspirin 250 mg and bolus injection of 5000 IU heparin immediately before start of trial. Patients in treatment group were given 100 mg recombinant tissue plasminogen activator over three hours (10 mg intravenous bolus, 50 mg during one hour, and 40 mg during next two hours) by infusion. Controls were given placebo by same method. Full anticoagulation treatment and aspirin were given to both groups until angiography (10-22 days after admission). beta Blockers were given at discharge. METHODS Left ventricular function at 10-22 days, enzymatic infarct size, clinical course, and survival to three month follow up. RESULTS Mortality was reduced by 51% (95% confidence interval -76 to 1) in treated patients at 14 days after start of treatment and by 36% (-63 to 13) at three months. For treatment within three hours after myocardial infarction mortality was reduced by 82% (-95 to -31) at 14 days and by 59% (-83 to -2) at three months. During 14 days in hospital incidence of cardiac complications was lower in treated patients than controls (cardiogenic shock, 2.5% v 6.0%; ventricular fibrillation, 3.4% v 6.3%; and pericarditis, 6.2% v 11.0% respectively), but that of angioplasty or artery bypass, or both was higher (15.8% v 9.6%) during the first three months. Bleeding complications were commoner in treated than untreated patients. Most were minor, but 1.4% of treated patients had intracranial haemorrhage within three days after start of infusion. Enzymatic size of infarct, determined by alpha hydroxybutyrate dehydrogenase concentrations, was less (20%, 2p = 0.0018) in treated patients than in controls. Left ventricular ejection fraction was 2.2% higher (0.3 to 4.0) and end diastolic and end systolic volumes smaller by 6.0 ml (-0.2 to -11.9) and 5.8 ml (-0.9 to -10.6), respectively, in treated patients. CONCLUSIONS Recombinant tissue type plasminogen activator with heparin and aspirin reduces size of infarct, preserves left ventricular function, and reduces complications and death from cardiac causes but at increased risk of bleeding complications4+