Contralateral carotid artery occlusion is thought to represent a significant risk factor in carotid endarterectomy (CEA). There is also evidence that intraoperative and postoperative hypotention may cause contralateral hemodynamic ischemia. As such, contralateral carotid artery occlusion is regarded as a risk factor for carotid angioplasty and stenting (CAS). In this paper, we report on five cases of severe ICA stenosis with contralateral carotid artery occlusions. Cerebral blood flow(CBF) and cerebral vasoreactivity( CVR) of the contralateral carotid artery occlusions were measured before and after CAS. Additionally, the influence that ipsilateral CAS exerted on the occluded side was examined. (123)I-IMP SPECT was performed before and after CAS, both at rest and at the time of acetazoramide administration. The CBF was evaluated quantitatively using the ARG method. The mean CBF of the treated side rose from 30.0 +/- 7.1 ml/100g/min to 34.4 +/- 8.3 ml/100g/min (p < 0.05), and the mean CBF of the occluded side similarly rose from 28.3 +/- 6.1 ml/100g/min to 31.7 +/- 6.4 ml/100g/min (p < 0.05). Correspondingly, the regional CVR (rCVR) increased from 5.9% +/- 16.3% to 35.0% +/- 16.4%(p < 0.05) on the treated side, and from 3.7% +/- 14.7% to 10.7% +/- 16.9% (p < 0.05) on the occluded side. This demonstrates that ipsilateral CAS seems to improve both CBF and CVR on the contralateral occluded side. The fact that some cases developed cross flow from the anterior communicating artery was both remarkable and significant. Where there was poor cross flow from the anterior communicating artery, improvement in cerebral vaso reactivity was limited.