Dipyridamol is a vasodilator that is used primarily in clinical practice as an antiplatelet agent. It increases coronary blood flow and was originally introduced as an antianginal agent. An ability to prolong a shortened platelet survival has been used to justify its value in preventing thromboembolic complications. Conditions characterized by a reduction in platelet survival and where dipyridamole has been used include heart valve replacement, arterial grafting, cerebrovascular disorders, and disorders of peripheral circulation. The in vivo effect of dipyridamole on platelet aggregation has not been well defined and may depend on additional factors. Prostaglandins appear to have important roles in platelet homeostasis; their relationships to the action of dipyridamole are discussed. Dipyridamole usually is combined with aspirin for synergistic anti-aggregatory purposes. However, the nature of the interaction has not been elucidated and benefit from the addition of dipyridamole has not been demonstrated in clinical studies. A review of clinical studies using dipyridamole indicates that it currently has limited value.