The natural history of intracranial arterial stenoses remains relatively unknown. To monitor the progression of these lesions over time, the authors reviewed transcranial Doppler (TCD) laboratory reports at five hospitals for patients with angiographically documented intracranial arterial stenoses along the internal carotid artery distribution, and at least two TCD studies conducted more than 2 months apart. Twenty-two patients (19 men and 3 women; mean age, 64 years) with 29 stenoses were identified. The findings were compared to reproducibility data obtained from 11 age-matched control subjects with repeat TCD studies. During a mean follow-up period of 21 months, peak systolic flow velocities corresponding to the areas of stenosis increased in 9 arteries with lesions, and new collateral flow patterns, indicating further hemodynamic compromise distal to the lesions, developed in 2; one of the latter also had increased corresponding velocities. Thus, 10 (35%) arteries with lesions had TCD evidence of progression. Flow velocities remained the same in 13 (45%) stenotic vessels and dropped in 2 (7%). Findings were considered inconclusive for 4 lesions (14%). These findings suggest that intracranial arterial stenoses are dynamic lesions, and that they can evolve and cause further reductions of the arterial diameters after relatively short periods of time. TCD can noninvasively detect their hemodynamic effects.