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Direct bypass surgery improves blood flow in patients with moyamoya disease (MMD) as well as causes local flow relocation and disturbance. This study aimed to describe the characteristics and changes of quantitative blood flow in MMD patients. Sixty-four consecutive MMD patients (67 hemispheres) who underwent STA-MCA anastomosis between August 2013 and September 2017 were included in this study. Intraoperative blood flow of superficial temporal artery and middle cerebral artery was measured with a noninvasive, ultrasound flowprobe, before and after the bypass. All data were collected prospectively. Before the bypass, 67 hemispheres were measured and 45 hemispheres acquired post-bypass measurements. Thirty-three (49.3%) had anterograde flow and 34 (50.7%) had retrograde. Retrograde blood flow was significantly higher than anterograde blood flow in MCA-M4 section (3.71 ± 2.78 ml/min vs 2.39 ± 1.71 ml/min, P = 0.023). Thirty-eight (56.7%) patients had local cerebrovascular reactivity, whereas 29 (43.3%) lost it. After bypass, blood flow was changed into three patterns: anterograde flow (1, 2.2%), retrograde flow (3, 6.7%), and bi-direction flow (41, 91.1%). Flow of M4 was significantly improved in both proximal (13.64 ± 9.16 vs 3.28 ± 2.57, P < 0.001) and distal (7.17 ± 7.21 vs 3.28 ± 2.57, P = 0.002) sides to the bypass. Postoperative infarctions occurred in 4 (8.9%) patients. All postoperative infarctions (n = 4) happened in those patients who did not have change of flow direction in M4 section distal to the bypass (P = 0.040). Direct bypass surgery improves local blood flow after bypass surgery in both sides of the anastomosis. Flow direction was usually changed after the surgery into a bi-direction pattern: anterograde flow in M4 distal to the bypass, and retrograde flow in M4 proximal to the bypass. Proximal side acquired around 5.4 times of flow than the distal side. Patients with change of flow direction in distal M4 were related with lower risks of postoperative infarctions. [ABSTRACT FROM AUTHOR]