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Context: Idiopathic nephrotic syndrome (INS) in children is a disease with considerable morbidity, yet the incidence and risk for relapse have not been systematically reviewed., Objective: To estimate the overall pooled weighted incidence and risk for relapse of INS in children., Data Sources: Medline and Embase (until December 2020)., Study Selection: All studies reporting incidence (per 100 000 children per year) and/or risk for relapse (the proportion of patients who experience ≥1 relapse) of INS in children (age: <18 years) were eligible., Data Extraction: After quality assessment, data were extracted: study (design, localization, and sample size) and patient (age, sex, steroid response, and ethnicity) characteristics, incidence, and risk for relapse., Results: After screening, 73 studies were included for analysis (27 incidence, 54 relapse). The overall pooled weighted estimate and corresponding prediction interval (PI) of the incidence was 2.92 (95% PI: 0.00-6.51) per 100 000 children per year. Higher incidences were found in non-Western countries ( P < .001). Incidence tended to be lower in white children, but this was not significant. The overall pooled weighted estimate of the risk for relapse was 71.9% (95% PI: 38.8-95.5). Between 1945 and 2011, incidence did not change ( P = .39), yet the risk for relapse decreased significantly ( P = .024), from 87.4% to 66.2%., Limitations: There was no full-text availability ( n = 33), considerable heterogeneity, and limited studies from Africa, Latin America, and Asia., Conclusions: INS has a low incidence with ethnic variation but high risk for relapse. Although corticosteroids have significantly reduced the risk for relapse, it remains unacceptably high, underscoring the need for alternative treatment strategies., Competing Interests: POTENTIAL CONFLICT OF INTEREST: The authors have indicated they have no potential conflicts of interest to disclose., (Copyright © 2021 by the American Academy of Pediatrics.)