Platelets play a major role in arterial thrombus formation - the cause of cardiovascular death, acute myocardial infarction and ischemic stroke and the number one killer in the Western World. Binding the COX enzyme platelet aggregation is inhibited by aspirin, and as prophylaxis against death, myocardial infarction and stroke aspirin is the most widely used drug worldwide. Low dose aspirin has been shown to improve outcome in patients with ischemic heart disease, but approximately 12% of these patients will suffer from a vascular event during a 2 year follow-up period despite aspirin therapy. It has been speculated that these patients might be "resistant" to the antiaggregatory effect of aspirin, and a diminished response to aspirin has been correlated with a less favourable outcome. However, at present no international consensus exists on how to measure "aspirin resistance".