The hypothesis that aspirin resistance is often due to noncompliance was investigated. One hundred ninety patients with a history of myocardial infarction were evaluated using arachidonic acid-stimulated light aggregometry at 3 different time points: while receiving their usual daily aspirin, after not receiving aspirin for 7 days, and 2 hours after the observed ingestion of aspirin 325 mg. At the first time point, 17 patients (9%) failed to show aspirin inhibition of platelet aggregation, but 2 hours after observed aspirin ingestion, aspirin inhibition was observed in all but 1 patient.