Angina and ischemic electrocardiographic changes occurred after administration of oral dipyridamole in four patients awaiting urgent myocardial revascularization procedures. To our knowledge, this has not previously been reported as a side effect of preoperative dipyridamole therapy, although dipyridamole-induced myocardial ischemia has been demonstrated to occur in animals and humans with coronary artery disease. Epicardial coronary collateral vessels were demonstrated in all four patients; a coronary "steal" phenomenon may be the mechanism of the dipyridamole-induced ischemia observed.