The importance of intraluminal coronary artery thrombus in acute myocardial infarction is now recognized. Coronary thrombi, however, may be important in ischemic syndromes other than infarction. The coronary angiograms of 268 consecutive patients undergoing diagnostic angiography were prospectively examined for intracoronary thrombus and form the basis of this study. Of these patients, 29 (11%) (25 men and 4 women) met the criteria for coronary artery thrombus. Of the 29 patients with thrombus, 24 (83%) had unstable angina before angiography. The five remaining patients with thrombus had had a transmural myocardial infarction 3 to 18 months before cardiac catheterization. In 21 patients, the thrombus was distal to a significant stenosis; in 8 it was proximal to or at the site of a significant stenosis. Coronary artery thrombus was identified in 24 (35%) of 67 patients with unstable angina compared with only 5 (2.5%) of 201 patients with stable angina (p less than 0.0001).