The prevalence of atherosclerosis at the carotid bifurcation and in the siphon was reviewed in 141 patients who underwent 149 endarterectomies. The relationship between the presence and severity of siphon lesions and focal neurologic symptoms, both before and after operation, was also examined. Siphon disease was found in 84% of the 282 sides. Most lesions (42%) were in the 20% to 49% diameter-reduction category. Only 9% were stenoses greater than 50%, and 10% were occlusions. The majority (65%) were smooth. No relationship was found between the severity of disease at the carotid bifurcation and in the siphon. No pattern of siphon disease could be related to the occurrence of symptoms. Furthermore, no relation was found between the severity of siphon disease and recurrent symptoms after endarterectomy.