OBJECTIVE: To elucidate the clinical and angiographic features in patients with moyamoya disease (MMD) and the p.R4810K heterozygous variant and present an angiographic grading system to evaluate disease severity. METHODS: We retrospectively reviewed 87 patients with MMD and the p.R4810K variant treated at Beijing Tiantan Hospital. Clinical features, stroke subtype, and angiographic characteristics were analyzed. RESULTS: The median age at diagnosis was 25 years (range, 3-59). The ratio of women to men was 1.2:1. The familial occurrence of MMD was 14.9%. The primary symptom at diagnosis was ischemia, hemorrhage, or other in 67, 16, and 4 patients, respectively. Angiographic features correlating with ischemic stroke or stroke, including Suzuki grade, external carotid artery collaterals, leptomeningeal collaterals, and Mugikura grade, were identified. A binary logistic regression model demonstrated a significant correlation of Suzuki grade (P = 0.008) and posterior cerebral artery grade (P = 0.029) with ischemic stroke (142 hemispheres). A modified Suzuki-Mugikura grading system was developed. The areas under the receiver operating characteristic curves used to predict ischemic stroke based on the Suzuki grading, Mugikura grading, and modified Suzuki-Mugikura grading systems were 0.736, 0.69, and 0.741, respectively. Furthermore, the modified Suzuki-Mugikura grades were significantly correlated with infarction in posterior circulation and the number of infarcted regions. CONCLUSIONS: The clinical and angiographic features of a Chinese MMD population with the p.R4810K variant were similar to those of a Japanese MMD population; they might be a distinct cerebrovascular disease entity and represent a separate subgroup. A modified Suzuki-Mugikura grading system was valuable for predicting stroke and evaluating disease severity.