Circadian rhythms have long been recognized to occur in many biologic phenomena, including secretion of hormones and activities of the autonomic nervous system. More recently, circadian rhythms have also been noted to occur in the incidences of certain cardiac and cerebrovascular events, including transient myocardial ischemia, myocardial infarction, sudden cardiac death, and stroke. The pathophysiology and the mechanisms underlying these variations are the focus of much investigation. The effects of different drug treatments on these circadian rhythms are also being studied. This information should enable better treatment strategies to be planned for patients who have either silent or symptomatic episodes of transient myocardial ischemia and potentially to prevent the occurrence of sudden, catastrophic cardiac events.