Ischaemia following surgery for aneurysmal subarachnoid haemorrhage (SAH) was defined by clinical criteria. Two distinct groups of patient developing ischaemia were identified: (1) patients recovering from anaesthesia with a new deficit or (2) patients developing 'true' delayed ischaemia. The only significant difference between the groups was an excess of pre-existing vascular disease in the former group. The eventual outcome for patients developing an immediate postoperative deficit was not significantly different from the delayed ischaemia group. The 6-month outcome for patients without clinical ischaemia in the first 10 postoperative days was excellent. The potentially different aetiology of ischaemia in the two subgroups with clinical ischaemia identified in this study should be considered during the planning and in particular the statistical design of trials testing new treatments for clinical ischaemia following aneurysmal SAH.