OBJECTIVE Cerebral embolism is a common cause of stroke. Microembolic signals (MES) detected by transcranial Doppler represent ongoing embolisms, but the lack of reliable data about its clinical relevance hinders its widespread use in clinical practice. METHODS We prospectively monitored 114 consecutive acute ischemic stroke patients with middle cerebral artery (MCA) stenosis for MES. The signals on digital audio tape were analyzed by an independent observer who was blinded to all other data. All patients were followed-up for the occurrence of recurrent stroke or transient ischemic attack (TIA) in the indexed MCA territory. RESULTS MES was detected in 25 (22%) patients. The mean number of MES was 18 (range, 1 to 102). MES were more common in patients with severe stenosis (10/21, 48%) than in those with mild-moderate stenosis (4/26, 15%) (Pearson chi2 P=0.02). During follow-up for a mean of 13.6 months (range, 1 to 32), 12 (12%) patients had further ischemic events (10 strokes and 2 TIAs) in the affected MCA region during follow-up. Among these, 7 (58%) had recurred within 1 month (1 recurred within 1 week, the other 6 patients had recurrences in week 3 or 4 after discharge), 3 within 6 months, 1 within 6 to 12 months, and the remaining 1 recurred after 1 year. The presence of MES was the only predictor of a further ischemic stroke/TIA by Cox regression (adjusted odds ratio, 8.45; 95% CI, 1.69 to 42.22; P=0.01) even after controlling for age, sex, diabetes, hypertension, previous stroke, smoking, and acute treatment. CONCLUSIONS In acute stroke patients with MCA stenosis, MES predicts further cerebral ischemia. This procedure should be considered as part of routine investigation and might identify a group of patients who are most likely to benefit from antithrombotic treatment.