Coronary arteriography was performed before and after the intracoronary injection of nitroglycerin to determine the presence or absence of spasm in patients within the first 12 hours of acute myocardial infarction. Coronary arterial spasm was demonstrated in six of fifteen (40%) acute myocardial infarctions associated with coronary artery disease. In five of the six instances the interval from the onset of symptoms to arteriography was less than 6 hours. Spasm was superimposed on a high-grade atherosclerotic obstruction and was separated from the catheter tip by a segment of normal vessel in each instance. The coronary artery remained patent (following the initial relief of spasm) in two patients maintained on sublingual nitrates and heparin. Spasm, superimposed on an atherosclerotic obstruction, may be the primary event or a secondary occurrence in the pathophysiology of acute myocardial infarction. Catecholamines could play an important role in the early pathophysiology of acute myocardial infarction by producing spasm and/or platelet aggregation at the site of an atherosclerotic obstruction. A dynamic interaction between spasm, platelet aggregates and the atherosclerotic plaque may precede coronary thrombosis.