The validity of transcranial Doppler (tD) was evaluated for the identification of stenoses of the middle cerebral artery (MCA). Twenty patients with atheromatous stenosis of the proximal MCA of more than 30% (3 of these cases with stenosis of more that 75%) had repeated transcranial Doppler between 1975 and 1987 and conventional angiography. No patient had carotid stenosis nor embolus-producing cardiopathy. Ten patients had had infarcts or transient ischemic attacks (symptomatic group). Ten patients were asymptomatic (asymptomatic group). Cerebral infarcts in the MCA territory occurred in 1 patient of each group and 1 additional patient in each group had an infarct in another territory. Follow-up was possible in 15 patients. The stenosis remained unchanged in 14, occlusion occurred in 1. The results of transcranial Doppler were compared to those of angiograms in this series and in an additional series of 40 patients who had had a normal angiogram. Sensitivity of transcranial Doppler was 60% (75% when stenosis was over 50%). Specificity was 95%. Comparison with angiography findings showed, among causes of failure of tD, analysis of the artery over too short a segment or kinked artery of very long stenoses, or too decreased blood flow. The failure rate of tD due to failure of bone penetration by ultrasounds was 5%. This new reproducible, non-invasive method appears to be able to detect MCA stenoses of 50% or more and help in follow-up.