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Background & Purpose: The additional effects of indirect bypass on combined revascularization surgery in moyamoya disease (MMD) remain unclear. Our purpose in this study is to evaluate how direct and indirect bypass in combined revascularization change through long term follow up and to analyze the clinical factors or hemodynamic parameters involved in these changes.Methods: A retrospective survey was conducted with a sample size of 97 hemispheres from 69 consecutive patients (43 adults and 26 children) with MMD treated by combined revascularization. We used magnetic resonance angiography (MRA) to evaluate bypass development with the diameters of superior temporal artery (STA) for the direct bypass and middle meningeal artery (MMA) and deep temporal artery (DTA) for the indirect bypass. A multivariate logistic regression analysis was used to study the related multiple variables on bypass development.Results: Good indirect bypass indicated by MMA development was seen 78% of the adult hemispheres and 95% of the pediatric ones. Dual development of direct and indirect bypasses was most frequently observed (47% and 56% of the adult and pediatric hemispheres, respectively). When indirect bypasses developed, 28% of the STAs reduced in diameter reciprocally from the acute to the chronic phases after surgery. The development of indirect bypass was significantly better in the adult hemispheres at Suzuki stage 4 or greater (P=0.02; odds ratio 7.4, 95% confidence interval, 1.4-39.4). The disease onset type and the hemodynamic parameters were not associated with the development of surgical revascularization.Conclusions: Indirect bypass developed effectively in both adult and pediatric patients, even in the presence of direct bypass. The universal application of indirect bypass is considered an optimal surgical strategy to maximize revascularization in MMD.