Purpose: Moyamoya disease (MMD) is a cerebrovascular disease characterized by progressive bilateral stenosis of internal carotid arteries with compensatory perforating vessels formation at the base of the brain. Recently some studies have shown that the RNF213 gene is associated with MMD susceptibility. Furthermore, there was a study for clarifying the correlation between the RNF213 genotype and early-disease onset and severe type MMD. Thus, we aimed to demonstrate the relation of RNF213 and the disease onset and severity in Korean pediatric MMD patients. Methods: We reviewed retrospectively the medical record of 54 MMD cases, who had an analysis data of the R4810K mutation in the RNF213 gene, between September 2006 and April 2013 at the Department of Pediatrics and Pediatric neurosurgery, Seoul National University Children\'s Hospital. The correlation of RNF213 genotype and disease onset and severity were statistically analyzed. Results: The R4810K variant was identified in 48 of 54 MMD cases (88.9%), including 10 homozygotes (18.5%) and 38 heterozygotes (70.3%). Homozygotes represented a significantly earlier disease onset (median age 2.8 years) than heterozygotes or wild types (median age 5.7, and 8.3 years, respectively). Also, the rates of cases diagnosed with MMD before age 4 was higher in homozygotes, compared with other types (60%, 23.7% and 0%, p = 0.006). One of poor prognostic factors, infarctions at initial presentation were higher in homozygotes than in heterozygotes and wild types (70%, 44.7% and 16.7%, p = 0.038), but involvement of posterior cerebral arteries showed no significant differences among them. Conclusions: RNF213 mutation is associated with early disease onset of MMD and severe prognostic factor in Korean children. But, further studies and follow up are needed for determine the actual role of RNF213 mutation in MMD.