BACKGROUND: Microembolic signals (MES) have been implicated as an important mechanism of acute ischemic stroke in middle cerebral artery (MCA) stenosis. We conducted a prospective study to correlate MES and patterns of diffusion-weighted MR images (DWI). METHODS: A total of 211 consecutive patients with MCA territory ischemic symptoms within 24 hours of symptom onset were monitored for MES. Among them patients with acute ischemic lesion in MCA territory on DWI were included. MCA velocities on transcranial Doppler (TCD) examination, patterns and multiplicity of DWI lesions, and the degree of stenosis on MR angiography (MRA) were compared between MES positive and negative groups. RESULTS: MES were detected in 9 patients (17.3%) among 52 patients who fulfilled our inclusion criteria. The mean number of MES per 30 minutes was 9.11+/-5.94 (range, 1-19). Mean flow velocity of insonated ipsilateral MCA in MES positive group was higher than that of MES negative group (115.67+/-81.9 cm/s vs 58.07+/-23.5 cm/s, p= 0.032). The degree of MCA stenosis on MRA and the presence of MES had significant relationship (p=0.039), but there was no significant relationship between lesion multiplicity on DWI and the presence of MES (p=0.431). The frequency of symptom recurrence in the corresponding MCA territory during 90 days from the index stroke did not show statistical significance between two groups (p=0.681). CONCLUSIONS: Our results do not support the idea of direct causal relationship between MES and the mechanism of atherothrombotic MCA stroke. Further study is needed to identify whether the appearance of MES implicates a predictor for further ischemic events or a sign of recanalization.