BACKGROUND This study examined the importance of ultrasonic plaque morphology and its correlation to the presence of intraplaque hemorrhage and clinical implications. METHODS One hundred fifty-two carotid plaques associated with > or = 50% internal carotid artery stenoses in 135 patients who had carotid endarterectomies were characterized ultrasonographically into irregular/ulcerative, smooth, heterogeneous, homogeneous, or not defined. All plaques were examined pathologically for the presence of intraplaque hemorrhage. RESULTS The ultrasonic morphology of the plaques included 63 with surface irregularity (41%), 48 smooth (32%), 59 heterogeneous (39%), 52 homogeneous (34%), and 41 not defined (27%). Intraplaque hemorrhage was present in 57 of 63 (90%) irregular plaques and 53 of 59 (90%) heterogeneous plaques, in contrast to 13 of 48 (27%) smooth plaques and 17 of 52 (33%) homogeneous plaques (P < .001). Fifty-three of 63 (84%) irregular plaques and 47 of 59 (80%) heterogeneous plaques had transient ischemic attack (TIA)/stroke symptoms, in contrast to 9 of 48 (19%) for smooth plaques and 15 of 52 (29%) for homogeneous plaques (P < .001). CONCLUSIONS Irregular and/or heterogeneous carotid plaques are more often associated with intraplaque hemorrhage and neurologic events. Therefore, ultrasonic plaque morphology may be helpful in selecting patients for carotid endarterectomy.