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Childhood idiopathic nephrotic syndrome (NS) is defined by proteinuria and hypoproteinemia. The incidence of childhood idiopathic NS varies with age, race, residential areas, and social conditions. In Japan, its incidence was estimated to be 6.49 cases/100,000 children. Our study aimed to investigate the incidence, characteristics, and rate of relapse of idiopathic NS in Fukushima between 2006 and 2016. Overall, 158 children aged from 6 months to 15 years old (65.8% male) developed idiopathic NS (median age at onset, 5.3 years). The peak age at onset was three years. The average annual incidence of childhood idiopathic NS was 5.16 (range, 3.47-9.26) cases/100,000 children. The highest incidence was in 2011, which was the year of the Great East Japan Earthquake and nuclear power plant accident, and reportedly caused psychological distress in the children at the time. Conversely, the five-year birth cohort showed minor difference from 2008 to 2012. The rate of incidence in males aged < 5 years was thrice greater than in females of the same age and almost the same for males and females aged 11-15 years. Of 507 total relapses in 115 NS children, common triggers of relapses were steroid discontinuation or reduction and infection. The average annual incidence of childhood NS based on the Fukushima population was lower than previously reported in Japan, and the annual incidence has changed over an 11-year period. These changes may be affected by social or environmental factors, including mental stress associated with lifestyle changes after the disaster. [ABSTRACT FROM AUTHOR]