The aim of this prospective study was to determine whether aortic atherosclerotic plaques are associated with increased frequency of microembolic signals and stroke in patients who undergo coronary artery bypass grafting. A total of 69 such patients were monitored by transcranial Doppler ultrasonography for 30 minutes before and after surgery. To our knowledge, this study is the Ist in which in vivo pathologic analysis of aortic plaques was systematically performed - and microembolic signals monitored - before and after open-heart surgery. Plaques were assessed by transesophageal echocardiography and by biopsy of materials taken during surgery. The frequency of microembolic signals was evaluated with regard to the occurrence of postoperative stroke. In the preoperative phase, only 10 of 48 patients with aortic plaques had microembolic signals, and the mean count of microembolic signals was 3.2 +/- 1.2 per hour At the conclusion of 24 postoperative hours, 29 patients (42%) displayed such signals (mean count 9.8 +/- 3.1/h). Seven of the 48 patients (15%) with aortic atherosclerosis had cerebral ischemic events, but none of those with normal aorta (21 patients) experienced stroke during the postoperative phase. During postoperative monitoring, patients with stroke had higher microembolic-signal counts than did those with normal aorta (17.4 +/- 3.3/h vs 5.9 +/- 3.1/h; P <0.05). Our findings suggest that microembolic signals can be a marker of severe aortic atherosclerosis and that monitoring these signals should enable the application of appropriate surgical methods to coronary artery bypass patients who are at higher risk of stroke.