Background and Purpose -Carotid endarterectomy clearly benefits patients with symptomatic severe stenosis (SCS), but the risk of stroke is so low for asympto matic patients (ACS) that the number needed to treat is very high. We studied transcranial Doppler (TCD) embolus detection as a method for identifying patients at higher risk who would have a lower number needed to treat. Methods -Patients with carotid stenosis of ≥60%by Doppler ultrasound who had never been symptom atic (81%) or had been asymptomatic for at least 18 months (19%) were studied with TCD embolus detection for up to 1 hour on 2 occasions a week apart; patient s were followed for 2 years. Results -319 patients were studied, age (standard deviation) 69.68 (9.12) years; 32 (10%) had microemboli at baseline (TCD+). Ev ents were more likely to occur in the first year. Patients with microemboli were much more likely to have microembol i 1 year later (34.4 versus 1.4%; P < 0.0001) and were more likely to have a st roke during the first year of follow-up (15.6%, 95%CI, 4.1 to 79; versus 1%, 95%CI, 1.01 to 1.36; P < 0.0001). Conclusions -Our findings indicate that TCD -ACS will not benefit from endarterectomy or stenting unless it can be done wit h a risk < 1%; TCD+may benefit as much as SCS if their surgical risk is not hi gher. These findings suggest that ACS should be managed medically with delay of surgery or stenting until the occurrence of symptoms or emboli. Background and Purpose -Carotid endarterectomy clearly benefits patients with symptomatic severe stenosis (SCS), but the risk of stroke is so low for asympto matic patients (ACS) that the number needed to treat is very high. We studied transcranial Doppler (TCD) embolus Detection as a method for identifying patients at higher risk who would have a lower number needed to treat. Methods-Patients with carotid stenosis of ≥60% by Doppler ultrasound who had never been symptom atic (81%) or had been asymptomatic for at least Results -319 patients were studied, age (standard deviation) 69.68 (9.12) years; 18 years (19%) were studied with TCD embolus detection for up to 1 hour on 2 occasions a week apart; Patients with microemboli were much likely to have microemboli at 1 year later (34.4 versus 1.4%; P <0.0001) and were more likely to have a st roke dur ing the first year of follow-up (15.6%, 95% CI, 4.1 to 79; versus 1%, 95% CI, 1.01 to 1.36; P <0.0001). Conclusions -Our findings indicate that TCD-ACS will not benefit from The endarterectomy or stenting unless it can be done wit ha risk <1%; TCD + may benefit as much as SC if their surgical risk is not hi gher. These findings suggest that ACS should be managed medically with delay of surgery or stenting until the occurrence of symptoms or emboli.