Resting middle cerebral artery velocity (Vmca) as well as its rate of increase on breath holding was measured on both sides in 11 age matched controls (Group I), in six symptomatic patients with unilateral internal carotid artery occlusion (Group II) and in nine symptomatic patients with unilateral internal carotid stenosis of greater than 75% (Group III), using a transcranial doppler velocimeter. There was a significantly lower resting Vmca in Group II patients on the occluded side, but not on the stenosed side in Group III, but there was considerable overlap of results. However, during breath holding, the rate of increase of Vmca allowed better separation of the occluded side (Group II) from the contralateral normal side as well as from controls, and from the stenosed side in Group III. Also there was a lower rate of rise of Vmca on the stenosed side in Group III than controls. As a test in symptomatic patients with a relevant carotid occlusion, values of less than 2.4 mm/sec2 were found in 100% of patients and only 4.5% of controls. Reactivity to breath holding promises to be a useful test in the evaluation of symptomatic patients with extra cranial carotid artery disease.