Tashiro R, et al. - In this investigation, researchers clarified the incidence, clinical presentation, and risk factors of the watershed shift (WS) phenomenon following direct revascularization surgery for adult moyamoya disease (MMD). Among 74 MMD patients having superficial temporal artery-middle cerebral artery (STA-MCA) anastomosis for 78 affected hemispheres, 60 adult subjects including 64 hemispheres had serial quantitative cerebral blood flow (CBF) analysis by N-isopropyl-p-[¹²³I] iodoamphetamine single-photon emission computed tomography following revascularization surgery. After STA-MCA anastomosis for adult MMD, the incidence of the WS phenomenon was 10.9 %. The clinical outcome of WS phenomenon is favorable in general, but there could be a risk for perioperative cerebral infarction. In order to avoid surgical complications, such as local cerebral hyperperfusion (CHP) and cerebral ischemia, caused by the WS phenomenon, routine CBF measurement in the acute stage following revascularization surgery for adult MMD was recommended. Concomitant detection of the WS phenomenon with local CHP is clinically important because in the presence of the WS phenomenon it may be necessary to avoid blood pressure reduction to counteract local CHP.