In a prospective study, 214 consecutive patients considered not to be candidates for surgical intervention were evaluated by means of duplex scanning. Of the patients, 135 had no symptoms and 79 had a history of previous neurologic symptoms. In 139 sides duplex scanning demonstrated nonhemodynamic stenosis (lumen diameter reduction, less than 50%) and in 99 sides, hemodynamic stenosis (lumen diameter reduction, 50% or greater). Of the 238 carotid artery plaques examined, 167 were homogenous and 71 were heterogenous. During a mean follow-up of 34 months, 27 new focal neurologic deficits occurred. Patients with previous symptoms had a higher incidence of new deficits (18/79 vs 9/135) (p less than 0.01). The severity of the stenosis and the presence of a heterogenous plaque were statistically correlated with the occurrence of new deficits (p less than 0.001). Multivariate analysis showed that the ultrasonographic pattern and the severity of the stenosis were independent variables. We conclude that a heterogenous plaque should be considered an unstable plaque with the possibility of causing cerebral ischemia.