Unlike thrombolytic agents, there are conflicting data regarding the time-dependent effect of aspirin treatment on outcome in acute myocardial infarction (AMI). We sought to evaluate the impact of timing of aspirin administration (before vs after thrombolysis) on mortality of patients with AMI. Our study included 1,200 patients with ST elevation AMI treated with thrombolysis. Early (n = 364) versus late (n = 836) users were defined as those receiving emergency aspirin before versus after initiation of thrombolysis, respectively. Time (median) from symptom onset to initiation of aspirin treatment was significantly shorter in early versus late users (1.6 vs 3.5 hours; p <0.001). There were no significant differences between the 2 groups with respect to baseline clinical characteristics. Early aspirin users were more likely to develop reischemia, to be treated with beta blockers, to be referred to coronary angiography, percutaneous transluminal coronary angioplasty, or coronary artery bypass graft surgery. Early users experienced lower mortality at 7 days (2.5% vs 6.0%, p = 0.01), 30 days (3.3% vs 7.3%, p = 0.008), and 1 year (5.0% vs 10.6%, p = 0.002) than late users. This survival benefit persisted for patients with and without previous aspirin therapy or revascularization and after adjustment for baseline characteristics and therapies at 7 days (odds ratio 0.36, 95% confidence interval 0.15 to 0.79), at 30 days (odds ratio 0.39, 95% confidence interval 0.17 to 0.82), and at 1 year (odds ratio 0.41, 95% confidence interval 0.21 to 0.74). Our study proposes a time-dependent benefit from aspirin in patients with AMI treated with thrombolysis.