The contribution of the external carotid artery to cerebral blood flow in the presence of an internal carotid occlusion or severe stenosis is well documented. This study was undertaken in order to try and exploit the external carotid artery as a collateral pathway to avoid cerebral ischaemia during carotid surgery. The main problem is to ascertain when the external carotid artery is relevant to cerebral perfusion, and to assess if the insertion of a shunt from the common to the external carotid artery is a useful way of ensuring adequate cerebral perfusion in patients with cerebral ischaemia during carotid clamping. In order to do this, it was necessary to assay the haemodynamic role of the external carotid artery by means of a technique which monitors cerebral function in a reliable way. We tried to evaluate this possibility by an intra-operative haemodynamic study during carotid surgery in 35 patients operated on under local anaesthesia. The insertion of a shunt between the common and external carotid artery was able to reverse brain ischaemia during clamping in four of eight patients with a neurological deficit during temporary carotid occlusion. In selected cases therefore cerebral protection with an external carotid shunt might be a valuable adjunct in the performance of carotid surgery.