Background and Purpose:To determine the relationship between ulceration, thro mbus, and calcification of carotid artery atherosclerotic plaques and symptoms o f ipsilateral or contralateral stroke. Methods:We compared microscopic plaque m orphology from patients with and without stroke symptoms ipsilateral or contrala teral to the plaque. Plaques were characterized for ulceration, thrombus, and ca lcification. We analyzed plaques from 241 subjects: 170 patients enrolled in the Asymptomatic Carotid Atherosclerosis Study (ACAS) and 71 patients enrolled in t he North American Symptomatic Carotid Endarterectomy Trial (NASCET); 128 subject s had no history of stroke symptoms, 80 subjects had ipsilateral symptoms, and 3 3 had contralateral symptoms. Results:Plaque ulceration was more common in plaq ues taken from symptomatic patients than those without symptoms (36%versus 14% ; P < 0.001); frequency of ulceration was similar for plaques associated with ip silateral(34%) and contralateral (42%) symptoms. Thrombus was most common in p laques taken from patients with both ipsilateral symptoms and ulceration. The ex tent of calcification was unassociated with stroke symptoms. Conclusion:Carotid plaque ulceration and thrombosis are more prevalent in symptomatic patients. Ul ceration is more common in symptomatic patients regardless of side of carotid sy mptoms, whereas thrombus is associated with ipsilateral symptoms and plaque ulce ration.Preoperative identification of carotid ulceration and thrombus should lea d to greater efficacy of stroke prevention by carotid endarterectomy. Background and Purpose: To determine the relationship between ulceration, thro mbus, and calcification of carotid artery atherosclerotic plaques and symptoms of ipsilateral or contralateral stroke. Methods: We compared microscopic plaque m orphology from patients with and without stroke symptoms ipsilateral or contrala teral to the Plaques were characterized for ulceration, thrombus, and ca lcification. Thirty patients were enrolled in the Asymptomatic Carotid Atherosclerosis Study (ACAS) and 71 patients enrolled in t North American Symptomatic Carotid Endarterectomy Trial (NASCET); 128 subjects had had no history of stroke symptoms, 80 subjects had ipsilateral symptoms, and 3 had contralateral symptoms. Results: Plaque ulceration was more common in plaq ues taken from symptomatic patients than those without symptoms (36% versus 14%; P < 0.001); frequency of ulceration was similar for plaques associated with ip silateral (34%) and contralateral (42 The ex tent of calcification was unassociated with stroke symptoms. Conclusion: Carotid plaque ulceration and thrombosis are more prevalent in symptomatic patients. Ul ceration is more common in symptomatic patients regardless of side of carotid sy mptoms, but thrombus is associated with ipsilateral symptoms and plaque ulceration. Preoperative identification of carotid ulceration and thrombus should lea d to greater efficacy of stroke prevention by carotid endarterectomy.