Transcranial Doppler ultrasound of middle cerebral arteries (MCAs) was used to detect asymptomatic embolic signals in a prospective study in patients with symptomatic and asymptomatic carotid stenosis. Recording from each artery for 20 min, embolic signals were more common ipsilateral to symptomatic arteries (eight of 38) than ipsilateral to asymptomatic arteries (one of 28, P = 0.04), or than in the MCAs of age-matched normal controls (none of 52, P < 0.005). Recording a subgroup of patients revealed that previously embolic-signal negative symptomatic stenoses frequently became embolic-signal positive when recording was repeated on another day. Including all recording periods (mean time: symptomatic 36.2 min, asymptomatic 46.2 min) embolic signals were detected ipsilateral to 13 of 38 symptomatic stenoses but only one of 28 asymptomatic stenoses (P = 0.003). In symptomatic subjects with embolic signals, median number of signals per hour was three (mean 26). One month following carotid endarterectomy embolic signals were not detected except in one patient who continued to experience frequent amaurosis fugax; in this patient following aspirin both symptoms and embolic signals were abolished. These studies suggest asymptomatic embolic signals correlate with clinical risk. Outcome studies are required to determine whether embolic signal detection may allow prediction of stroke risk and monitoring of the effectiveness of therapy. The technique may also prove useful in studying the pathophysiology of cerebral embolism.