Aim: Previous studies have found significant differences in clinical characteristics between pediatric and adult moyamoya disease (MMD) patients, but few studies have focused on the factors underlying these differences. We aimed to investigate the differences in leptomeningeal collateral?(LMC) status between pediatric and adult MMD patients and to analyze the effects of LMCs on clinical characteristics and therapeutic prognosis. Methods: We retrospectively analyzed 214 MMD patients from January 2014 to January 2016. Clinical characteristics and LMC status were compared between the pediatric and adult patients. LMC status was graded as good or poor depending on the retrograde flow from the posterior cerebral artery (PCA) on digital subtraction angiography (DSA). Results: A total of 83 pediatric and 131 adult (1:1.6) MMD patients were analyzed. Pediatric patients were more likely to experience a transient ischemic attack (81%), whereas adult patients were more likely to experience infarction (51%). Regarding the different MMD stages (the early, medium, and advanced stages corresponded to Suzuki stages 1-2, 3-4, and 5-6, respectively), the prevalence of good LMC status was higher for pediatric patients than for adult patients in the early stage (P?=?0.047) and the medium stage (P?=?0.001), but there were no differences between these patient groups in the advanced stage (P?=?0.547). Worse postoperative angiographic outcomes (P?=?0.017) were found in adult patients than in pediatric patients in the medium stage. Poor LMC status had strong correlations with infarction (P?<?0.001 and P?=?0.017) and poor postoperative outcomes (P?=?0.003 and P?=?0.043) in both pediatric and adult patients. Conclusions: Pediatric MMD patients have greater patency and a greater ability to establish good LMC status than adult patients, and poor LMC status has a strong correlation with severe clinical symptoms and poor postoperative outcomes. LMC status may be an important factor in the differences in clinical characteristics and prognosis between pediatric and adult MMD patients. ? 2019 The Authors. CNS Neuroscience & Therapeutics Published by John Wiley & Sons Ltd.