Arterial thromboembolic disorders are the leading cause of death in most of the advanced nations. The study of epidemiologic relationship of these disorders to biological rhythms may lead to a better understanding and perhaps a better treatment. Chronoepidemiologic studies demonstrated a morning peak for arterial thromboembolic disorders (cerebral and myocardial infarctions) and sudden cardiac deaths. These variations might be explained partially by circadian variations in hemostasis. Indeed chronophysiologic studies have shown that hemostatic variables follow circadian rhythms. The level of platelet aggregation and that of blood coagulation have been found to be increased in the morning whereas fibrinolytic activity is lower at this time of the day. The facts suggest a chronotherapeutic approach in thromboembolic disorders.