We report the results of a prospective follow-up study which was designed to identify the sequential change of regional vasodilatory capacity and its relation to long-term prognosis in patients with atherothrombotic brain ischemia. We evaluated a total of 139 acetazolamide tests using 123I-IMP SPECT in 76 patients with occlusion or greater than 75% stenosis of the carotid or middle cerebral arteries. Reduced vasodilatory capacity was demonstrated in 34 patients (44.7%) at entry. Patients were divided into two groups on the basis of the initial acetazolamide test, i.e., impaired and non-impaired group. All patients were prospectively followed for 28.4 +/- 15.9 (Mean +/- S.D.) months and acetazolamide tests were repeated every 1-2 years. Follow-up acetazolamide tests demonstrated normalization of impaired vasodilatory capacity in 14 patients, including 12 without surgical revascularization. The survival analysis comparing the two groups for all strokes and all death, and for ipsilateral ischemic strokes and all death did not demonstrate the poor prognosis of imparied group. However, the recurrence of ischemic stroke was found in 4 out of 8 cases without improvement of vasodilatory capacity. The present data indicate that the sequential evaluation of vasodilatory capacity is of importance in the clinical management of patients with major cerebral artery occlusion or severe stenosis.