Among 662 consecutive carotid endarterectomies eight cases of postoperative ipsilateral intracerebral haemorrhage were identified, occurring into brain areas which, preoperatively were without infarction. As blood pressures across the stenosis were routinely measured during surgery, the internal carotid artery (ICA) perfusion pressure could be related to the occurrence of haemorrhage. In addition, cerebral blood flow (CBF) was studied with the intravenous xenon-133 technique in four patients and histopathologic examination of the brain was available in four patients who died subsequent to their haemorrhage. All eight patients had a high grade of ICA stenosis and a marked reduction of ICA perfusion pressure (average of 40%) which was significantly greater than that observed (average of 6%) in the other patients undergoing carotid surgery (P less than 0.0001). Relative hyperperfusion of the ipsilateral hemisphere was seen in the four patients studied postoperatively. In at least two cases the haematoma was preceded by an asymptomatic postoperative ischaemic infarct. Histologic examination did not confirm previous findings of changes resembling those seen in malignant hypertensive encephalopathy. These results substantiate the view, that patients at risk of haemorrhage after endarterectomy are those with a low preoperative cerebral perfusion pressure and postoperative hyperperfusion. Postoperative silent brain infarction is an additional risk factor.