OBJECTIVE : To determine the age- and sex-specific prevalence and incidence of demyelinating optic neuritis and the risk of multiple sclerosis (MS) in pediatric and adult populations in South Korea. METHODS : A nationwide, population-based, retrospective study using data from the Korean National Health Claims database from 2010 to 2016. METHODS : The entire South Korean population aged 65 years of age or younger (n = 44 700 564). All patients with optic neuritis from the entire Korean population were included. METHODS : Patients aged 14 years of age or younger were classified as pediatric patients, and those aged 15 to 65 years were classified as adults. Each group was analyzed separately. Patients with optic neuritis had a subsequent diagnosis, including idiopathic, MS, neuromyelitis optica (NMO), and acute disseminated encephalomyelitis. Prevalence and incidence, conversion rate to MS, and treatment modalities (steroids, plasmapheresis, interferon-β, and immunosuppressants) were estimated. METHODS : Prevalence and incidence of optic neuritis, and conversion rate to MS. RESULTS : Among 44 700 564 individuals, 531 pediatric patients (50.7% female) and 7183 adults (53.3% female) were identified as having optic neuritis. Annual incidence was 1.04 (95% confidence interval [CI], 1.01-1.07) per 100 000 pediatric individuals and 3.29 (95% CI, 3.28-3.30) per 100 000 adults. Peak incidence was observed at 10 to 14 years in the pediatric population and at 30 to 34 years and 50 to 54 years in the adult population. Conversion rate to MS was 13.8% in the pediatric population and 11.4% in the adult population. Fourteen percent of all patients were treated with chronic immunosuppressants, 38% of patients with NMO underwent plasmapheresis, and 50% of patients with MS were treated with interferon-β. CONCLUSIONS : This is a nationwide epidemiologic study of optic neuritis in individuals of all ages in South Korea. The incidence of optic neuritis and subsequent risk of MS in the pediatric population are comparable to those reported in western countries but are lower in the adult population than in western countries. The incidence rate in adults was 3.2-fold higher than in the pediatric population, and the overall MS conversion rate in the entire Korean population was estimated to be 10.6%.