Decreased oxygen supply rather than increased demand may be the primary pathogenic mechanism of myocardial ischemia in patients with unstable angina. Coronary artery spasm and in vivo platelet aggregation may play an important role in the mechanism by which the magnitude of fixed obstruction secondary to coronary atherosclerosis is transiently exacerbated. In this case report, we describe a patient who developed chest pain accompanied by ischemic ECG changes during coronary arteriography, due to a transient thrombotic aggravation of a fixed, significant coronary stenosis. Percutaneous transluminal coronary angioplasty was performed with relief of the coronary stenosis and of the symptoms.