Although there has been extensive research attempting to identify psychophysiologic mechanisms linking psychological events and behavior patterns with risk for coronary heart disease, these are important potential mechanisms that remain largely unexplored. One of these mechanisms is thrombosis, which is widely recognized as the precipitant of acute coronary events (myocardial infarction and sudden ischemic death). A major element in arterial thrombosis is the platelet, and platelet inhibition has been shown to be effective in preventing coronary events. In the present paper, platelet physiology and pathophysiology are reviewed. Platelet function testing and the clinical role of platelet activation in atherosclerosis and acute coronary events are also discussed. Because epinephrine and possibly shear stress are clinically important activators of platelets, it is proposed that platelet reactivity to psychological stress may be a major mechanism in coronary events. The literature to date supports the hypothesis that platelet activity is increased by emotional stress. Suggestions are made for future research in this area.