Dear Editor, The results of the previously published Carotid Occlusion Surgery Study (COSS) seem to influence worldwide the view and position towards the surgical treatment for ischaemic cerebrovascular disease [7]. Quite recently, a position statement by the Cerebrovascular Section of the American Association of Neurological Surgeons (AANS) and Congress of Neurological Surgeons (CNS) has been provided [2]. The goal of the present letter is to underline this statement and to provide additional insight on the position of the Cerebrovascular Section of the European Association of Neurological Surgeons (EANS) regarding the clinical impact of COSS. Historically, the EC-IC Bypass Trial published in the 1980s, failed to demonstrate a benefit of superficial temporal artery to middle cerebral artery bypass in more than 1,300 patients suffering from symptomatic internal carotid artery or middle cerebral artery atherosclerotic disease [9]. In the following years the study was widely criticised, predominately due to: (1) a large number of patients operated on at participating centres who were not included in the study (selection bias) and (2) the lack of patient selection based on haemodynamic assessment [5, 8]. Subsequently, in the 1990s, the analysis of haemodynamic impairment to predict the risk of stroke became more important and it was suggested to identify candidates for bypass surgery according to haemodynamic criteria. In COSS, the main inclusion criteria were: (1) transient ischaemic attack (TIA) or ischaemic stroke in the territory of an occluded carotid artery and (2) ipsilateral-to-contralateral hemispheric mean OEF ratios >1.13. A sample-size power calculation was based on the STLCOS data and it was estimated to require a total of 372 patients. The primary outcome was defined as all strokes and deaths within 30 days after surgery and ipsilateral ischaemic stroke within 2 years [8]. Based on an interim analysis, the study was preliminarily halted after inclusion of 195 patients due to the following observations: