Objectives. This analysis aimed to evaluate in a large patient cohort the relation between ST segment alterations after fibrinolytic therapy for acute myocardial infarction and 1) the combined end point of in-hospital mortality plus clinical congestive heart failure or extensive left ventricular damage, and 2) mortality 30 and 180 days after randomization.Background. Angina relief, enzyme release acceleration and ST segment normalization are related to coronary artery reperfusion and prognosis. Electrocardiographic (ECG) evaluation before and after fibrinolytic drug administration has been used to predict short-and long-term clinical outcome in acute myocardial infarction.Methods. Patients enrolled in the Gruppo Italiano per lo Studio della Sopravvivenza nell'Infarto Miocardico (GISSI-2) trial underwent a standard ECG on admission and after 4 h of alteplase or streptokinase therapy; 7,426 recordings were suitable for ST segment analysis. A decrease ≥ 50% in the sum of ST segment elevation in all ECG leads was adopted as the cutoff for predicting coronary artery patency. Recanalization was deemed to have occurred in 4,951 patients (group A) versus 2,475 patients without reperfusion (group B).Results. Group A patients experienced a lower incidence of the combined end point than did group B patients (16.2% vs. 22.9%, respectively), as well as of all its components (death, clinical heart failure, ejection fraction < 35%, injured myocardial segment > 45%, QRS score > 10). Thirty- and 180-day mortality rates were lower in group A than group B (3.5% and 5.7% vs. 7.4% and 9.9%, respectively); relative risk (Cox) was 0.46 (95% confidence interval [CI] 0.37 to 0.57) for 30-day and 0.58 (95% CI 0.48 to 0.70) for 180-day mortality. Patients in group A had significantly less ventricular fibrillation and sustained ventricular tachycardia but more ischemic episodes (early recurrent angina plus myocardial infarction recurrence).Conclusions. A simple, inexpensive instrumental evaluation, unaffected by different epidemiologic and clinical characteristics of the population analyzed, can allow early assessment of the effectiveness of fibrinolytic treatment with respect to the main clinical outcomes.