There has been limited data on the frequency of microembolic signals in patients with middle cerebral artery (MCA) stenosis, especially during the acute phase of stroke. Using transcranial Doppler, the authors prospectively monitored the MCA segments distal to stenosis in 4 groups of patients for 30 minutes: (1) symptomatic patients with acute ischemic stroke and MCA stenosis, (2) asymptomatic group patients with asymptomatic MCA stenosis, (3) control patients with acute ischemic stroke of undetermined etiology, and (4) normal people. A total of 60 patients completed the study. There were no microembolic signals in the asymptomatic, control, and normal groups. Among 20 patients in the symptomatic group, microembolic signals were detected in 3 patients (15%). The number of emboli ranged from 1 to 6 per 30 minutes. This is the first report of the presence of microembolic signals in acute stroke patients with MCA stenosis.