Prognosis in acute myocardial infarction has been compared in patients with and without diagnostic ECGs. Of 817 patients, 89.4% had ST elevation, 2.4% had left bundle branch block, and 8.2% had no ST elevation. Patients without ST elevation had a hospital mortality rate of 3.0% compared with 14.0% and 40.0%, respectively, in patients with ST elevation and left bundle branch block (p = 0.0001). Event-free survival at 6 months in patients without ST elevation was 85.6% (74.1% to 92.3%), compared with 72.9% (69.4% to 76.0%) and 31.0% (12.0% to 52.3%) in patients with ST elevation and left bundle branch block (p < 0.001). The excess risk associated with ST elevation was largely attributable to the severity of infarction: after adjustment for Q-wave development and heart failure, the hazard ratio fell from 2.24 (1.43 to 4.38) to 1.76 (0.86 to 3.59). In conclusion, acute myocardial infarction has a considerably better prognosis when it is unassociated with ST elevation or left bundle branch block. This finding may have important implications for interventional management.