Silent myocardial ischemia is a marker in patients with coronary artery disease identifying those at high risk for subsequent cardiac events. During provoked myocardial ischemia some patients with angina pectoris do not develop chest pain. Are there clinical, angiographic or electrocardiographic differences between patients with chest pain as compared with patients without chest pain during provoked myocardial ischemia? Coronary angioplasty is a well-established method for the treatment of coronary stenosis, but it is also an interesting model for the study of myocardial ischemia as a result of coronary occlusion. We monitored 114 patients with angina pectoris during coronary angioplasty with dynamic, computerized vectorcardiography. During inflation of the balloon 33 of 114 patients had silent ischemia. Patients with silent myocardial ischemia had similar reasons for terminating the preangioplasty exercise test and where on similar anti-ischemic drug regimes. Silent myocardial ischemia was significantly associated with a history of diabetes, presence of collaterals, a history of less severe previous angina and less ST segment changes during angioplasty as compared with patients with painful ischemia. It is suggested that during coronary angioplasty silent ischemia may be caused by a less severe degree of ischemia, possibly as a result of the protective effect of collaterals.