A prospective analysis of 296 carotid arteries in 293 asymptomatic patients was undertaken using real-time B-mode ultrasonography. All patients had carotid bifurcation disease and were followed for an average of 46 months. The endpoint for follow-up was a transient ischemic attack or stroke. Patients were categorized according to degree of stenosis (greater or less than 75 percent) and morphologic plaque characteristics (calcified, dense, or soft). Patients with hemodynamically significant stenosis were at greater risk of transient ischemic attack or stroke than their counterparts with less than 75 percent stenosis. However, even patients with less than 75 percent stenosis were at risk if the plaque was less organized (dense or soft). Patients with hemodynamically significant stenosis and morphologically soft plaque were at the greatest risk of transient ischemic attack or stroke. Those patients with calcified plaque and less than 75 percent stenosis had the lowest risk of transient ischemic attack or stroke. Morphologic plaque characteristics, as well as degree of stenosis, is important in determining which patients are candidates for carotid endarterectomy.