Objectives: To date, risk factors for poor renal prognosis in patients with lupus nephritis (LN) include sex, race, presence of antiphospholipid antibodies, renal function at diagnosis, frequent nephritic flares, and prolonged proteinuria. However, the influence of male sex on long-term renal outcome in childhood onset lupus nephritis is controversial. Our objective was to analyze the clinical features and course of childhood onset LN in South Korea and to determine male sex as a risk factor for poor renal prognosis. Methods: The Korean Society of Pediatric Nephrology has organized a retrospective cohort study of childhood onset LN centered on 13 major pediatric nephrology centers in South Korea. The study included 254 patients histologically diagnosed with LN. Results: The median age at renal biopsy was 13.1 years and median follow-up period was 7.6 years. The main forms of presentation were nephrotic syndrome in 175 patients (74.5%), and WHO class IV LN in 166 patients (65.4%). After 6 months of induction treatment, 150 patients (68.2%) achieved complete renal remission. During the follow-up, 18 patients (7.3%) started renal replacement therapy (RRT). When men and women were compared, there were significant differences in several variables (p<0.05). Low complement 4 levels at diagnosis, prolonged proteinuria, and an estimated glomerular filtration rate (eGFR) of less than 60 ml/min per 1.73 m2 at last visit. In multivariate analysis, male sex was an independent predictor of long-term renal prognosis: prolonged proteinuria (adjusted OR 2.209) and eGFR of less than 60 ml/min per 1.73 m2 (adjusted OR 2.686). There were no significant differences between men and women in the risk of initiating RRT during follow-up. Conclusions: In this study, childhood-onset LN usually appears as nephrotic syndrome, and WHO class IV is the most common form in South Korea. When men and women were compared, male sex was associated with a worse long-term renal prognosis.