Objectives: The purpose of our study was to determine the frequency of aspirin administration among patients with acute myocardial infarction (MI) as dictated by physicians practicing in the United States. Background: Aspirin (ASA), a widely available, inexpensive and generally well-tolerated platelet inhibitor, is recommended for patients with acute coronary syndromes, including acute MI. However, there is concern that aspirin is underutilized in daily clinical practice. Methods: Early (< 24 hours) and predischarge ASA administration were determined among 220,171 patients with suspected acute MI enrolled in the Second National Registry of Myocardial Infarction (NRMI 2) between June, 1994 and April 30, 1996. Results: Overall, 165,122 (74.9%) of patients received ASA within 24 hours of hospital admission, whereas 55,049 patients did not. Early ASA recipients were younger, more often male, arrived at the hospital earlier, and were more likely to be classified as Killip Class II or less compared to those who did not receive ASA. Patients who received aspirin were also more likely to have chest pain, electrocardiographic ST segment elevation, and tended to arrive at the hospital earlier than those who did not receive ASA. However, over 20% of patients with ST segment elevation did not receive early ASA therapy. From the total cohort of early ASA recipients, only 69% received ASA at the time of hospital discharge. Trends in early and pre-discharge aspirin administration over a 2 year time period in all patients (72.6 to 75.1% and 71.5 to 74.6%, respectively; p < 0.001) and in specific patient subsets were encouraging with a gradual but steady increase; however, utilization remained comparatively low in women and the elderly. By multivariable analysis, in-hospital recurrent MI (OR 0.90, 95% CI; .78-1.0, p = 0.04), stroke (OR 0.65, 95% CI, .52-.80, p < 0.001) and death (OR 0.24, 95% CI, .22-.26, p < 0.001) occurred less frequently when ASA was administered within 24 hours of hospitalization. Conclusion: Aspirin is currently underutilized in routine clinical practice as both primary and adjunctive forms of therapy in MI, especially among patients known to be at risk for recurrent cardiothrombotic events. The targeted and timely use of aspirin reduces early cardiovascular events and should remain a priority in national health care efforts.