A group of 30 patients with symptomatic internal carotid artery (ICA) occlusion and different computed tomography (CT) patterns of cerebral ischemia (14 borderzone and/or terminal, 16 territorial) were studied by means of transcranial Doppler (TCD) to determine the relationship between vasomotor reactivity and the topography of the infarct on CT scan. The vasomotor reactivity was evaluated by assessing the mean flow velocity changes observed in the middle cerebral artery (MCA) after the longest possible apnea and i.v. Diamox injection (12–15 mg/kg body weight). An impairment in the vasomotor response was found in 13 out of 14 patients with borderzone and/or terminal ischemia but in only 1 out of 16 with complete territorial ischemia. No relationship with the basal MCA flow velocity was observed. Diamox and apnea tests in TCD examination can be considered useful tools for investigating the hemodynamic changes following ICA occlusion.