Angiographic assessment of the culprit coronary artery lesion before acute myocardial infarction. Serial angiographic studies of patients with myocardial infarction and unstable angina suggest that the culprit plaque underlying a thrombus need not have produced severe luminal obstruction before onset of the event. An atherosclerotic coronary artery lesion can, therefore, have 2 important characteristics. First, it may be obstructive. Second, it may be "vulnerable" in that it has the potential to become thrombogenic if exposed to the appropriate triggering stimulus. A lesion need not be obstructive to become thrombogenic, nor do all obstructive lesions have thrombogenic potential. The cause of an infarction may thus be rupture of a nonobstructive plaque leading to occlusive thrombus formation. Because it may be difficult to predict the site of a subsequent occlusion from a coronary angiogram, coronary bypass surgery or angioplasty directed only at discernible stenotic lesions may not be effective for preventing subsequent myocardial infarctions. Appropriate therapy may need to be directed at the entire coronary tree. Such therapy might include cholesterol lowering, beta blockade and aspirin. 