The mechanism by which carotid bifurcation atheroma produces transient neurologic dysfunction has been explained by the arterial stenotic and the arterial embolic theories. There are several observations, however, that cannot be explained by the stenotic theory alone. The embolic theory is a better explanation, but has been difficult to prove because the criterion for operative selection is a high grade stenosis. Relief of symptoms, therefore, could be due either to improved blood flow or removal of an embolic source. To separate the factor of improved blood flow, we have performed 49 carotid endarterectomies for nonstenotic, ulcerative lesions of the carotid bifurcation. Our patients have been totally relieved of hemispheric symptoms when followed for up to 39 months, suggesting that the cause of their transient ischemic attacks was the release of emboli from the carotid bifurcation.