The importance of the patency of the contralateral carotid artery on the haemodynamic effect of a carotid stenosis was investigated. The breath-holding index was used to estimate cerebral perfusion reserve. The percentage rise in middle cerebral blood flow velocity, measured by transcranial Doppler ultrasonography, occurring during breath-holding was divided by the time of breath-holding. In study 1, ten subjects with no carotid stenosis, 13 with unilateral and eight with bilateral stenosis were studied. For unilateral stenosis the breath-holding index was reduced on the stenosed side compared with normal (0.85 versus 1.34 per cent per s, P < 0.05). For bilateral disease, in the presence of severe stenosis on the side contralateral to that being examined, even a moderate stenosis (30-50 per cent) resulted in a much greater reduction in the breath-holding index. In study 2, the breath-holding index was measured before and after carotid endarterectomy in ten patients. The index increased significantly following operation, not only on the operated side (1.31 versus 0.79 per cent per s, P < 0.01), but also on the contralateral side (1.23 versus 0.87 per cent per s, P < 0.05). In three of four patients with contralateral stenosis > 90 per cent or occlusion, operation improved the breath-holding index markedly (> 90 per cent) on the non-operated side above the stenosis or occlusion. These results demonstrate the importance of patency of the contralateral carotid artery in determining the haemodynamic effect of a carotid stenosis. In addition, operating contralateral to an occlusion may improve haemodynamic status above the occlusion. Implications in selecting patients for carotid endarterectomy are discussed.