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Venous oxygen saturation (SO2) is measured in medical fields to assess tissue circulation insufficiency. This study aimed to elucidate the use of a cortical venous redness measurement to evaluate hemodynamic changes during revascularization surgery for patients with moyamoya disease.In this retrospective case-series analysis, we first quantitatively measured and correlated SO2 and R intensity of 24-bit color digital red–green–blue pictures of blood samples from 3 volunteers. Subsequently, based on intraoperative digital pictures of 29 patients with moyamoya disease, we measured the R intensities of a cortical vein near the anastomosis site before and after anastomosis. Cerebral blood flow (CBF) at the site was measured using a single-photon emission computed tomography before and 1 to 3 days after surgery. Venous R intensity and CBF were measured twice by 4 raters, and their correlations were examined using generalized linear mixed effect model and linear regression analysis.A strong linear correlation was found between blood R intensity and its SO2 (coefficients, 0.522; 95% confidence interval, 0.364–0.680, using generalized linear mixed effect model). Venous R intensity before the anastomosis was not correlated with preoperative CBF (coefficients, 0.000352; 95% confidence interval, −0.000369 to 0.00107, by generalized linear mixed effect); however, the increases in venous R intensity after anastomosis were correlated with postoperative increases in CBF (R2, 0.367; 95% confidence interval, 0.116–0.618 to 0.548; 95% confidence interval, 0.331–0.764, by linear regression analysis).Cortical venous redness represented impaired CBF and could be a useful parameter for assessing hemodynamic changes during revascularization surgery.