Background The aim of this article was to investigate whether prior aspirin use in patients with acute coronary syndromes affects clinical outcome. The Efficacy Safety Subcutaneous Enoxaparin in Non-Q-Wave Coronary Events Study (ESSENCE) and Thrombolysis in Myocardial Infarction (TIMI) 11B trials have shown superiority of enoxaparin over unfractionated heparin (UFH) in patients with unstable angina and non-ST-segment elevation myocardial infarction (UA/NSTEMI). However, the treatment effect of enoxaparin in the subset of patients reporting prior aspirin use has not been determined. Methods The rate of death, myocardial infarction, and urgent revascularization at days 8 and 43 after randomization was compared among patients who received aspirin within the week before randomization with those who did not receive aspirin in the TIMI 11B trial. A total of 3275 patients (84%) were prior aspirin users. Results The admission diagnosis was similar for prior and nonprior aspirin users. At both day 8 and day 43 the event rate was higher for prior aspirin users than for nonprior aspirin users (odds ratio 1.6 [1.24-2.08], P = .0004 at day 43), even after correction for baseline characteristics. Compared with those prior aspirin users taking UFH, enoxaparin-treated prior aspirin users had a reduced rate of the composite end point of death, myocardial infarction, and urgent revascularization at day 8 (odds ratio 0.82 [0.67-1.00], P = .046) and day 43 (odds ratio 0.83 [0.70-0.98], P = .032). Conclusion Patients with UA/NSTEMI and prior aspirin use had a 60% higher risk of death and cardiac ischemic events compared with nonprior aspirin users. On the basis of this subanalysis, enoxaparin is superior to UFH in all patients. In prior aspirin users the benefit is more clearly demonstrated. (Am Heart J 2001;141:566-72.)