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 [1]. In the following years, the study was widely criticized, predominantly due to (1) the large number of patients operated on at participating centers who were not included in the study (selection bias), and (2) the lack of patient selection based on hemodynamic assessment [2, 6]. Subsequently, in the 1990s the analysis of hemodynamic impairment to predict the risk of stroke became more important and it was recommended to identify candidates for bypass surgery according to hemodynamic criteria.