Background: Surgical cerebral revascularization is recommended for treating pediatric moyamoya disease (MMD). However, whether unilateral combined bypass surgery can cause disease progression on the contralateral side is uncertain. The study aimed to investigate the vascular architecture and regional cerebral blood flow (rCBF) status of patients with pediatric MMD after successful unilateral combined bypass surgery and to identify the possible risk factors. Methods: Pediatric patients diagnosed with MMD and admitted to Xuanwu Hospital who underwent combined bypass surgery between 2019 and 2021 were enrolled. Digital subtraction angiography (DSA) and magnetic resonance imaging (MRI) with arterial spin labeling (ASL) were performed to investigate the vascular architecture and rCBF during surgery and at short-term follow-up. Suzuki's angiographic staging and moyamoya vessel grading system were both used. Progression was defined as an increase in either Suzuki stage or moyamoya vessel grade detected after unilateral surgery. All analyses were performed with conventional statistic methods. Results: A total of 27 successive patients with a median age of 8 [interquartile range (IQR), 5-14] years old were identified. On the non-operated (non-OP) side, 11 (40.7%) patients demonstrated progression, all of whom showed an increase in the moyamoya vessel grade, and 5 also displayed Suzuki stage progression during the median 4.7 (IQR, 3.7-5.7) months follow-up. However, rCBF barely changed on the non-OP side compared to preoperation (preoperation: median, 49.6, IQR, 42.9-61.1 mL/100 g/min; postoperation: median, 50.2, IQR, 43.5-59.3 mL/100 g/min; P=0.445). Conclusions: Combined bypass surgery might accelerate the radiological progression on the contralateral side, which occurs before the decline of rCBF. Those with earlier Suzuki stage MMD of the non-OP side are prone to rapid progression after unilateral combined revascularization.