Background: The association between HDs and children’s interstitial lung disease (ILD) were firstly recognized on 2011, thereafter over 2,000 persons were identified as HDs victims. On November 11, 2011, humidifier-disinfectants (HDs) were permanently banned in South Korea. Asthma has become one of the major diseases appealed by victims. Objectives: The study aimed to investigate the change of the incidence and prevalence of asthma and ILD in children in the nation-wide level, and to analyze the policy effect of stopping sales of HDs. Methods: We conducted nationwide, population-based, retrospective birth cohort study using data from Health Insurance Review and Assessment (HIRA). Children under 6 years of age between 2008 to 2015 in Korea, were included and reviewed up to end of 2015. The primary outcome was the time-trends of asthma-prevalence, and comparison of asthma-related health-outcomes before and after the stopping sales of HDs. Secondary outcome was to investigate the changes of incidence of interstitial lung disease and asthma. We defined asthma (J45, 46) and ILD (J841, 848, and 849). using KCD-5, 6 Results: We analyzed 817,809 children with asthma between 2008 and 2015. After stopping sales of HDs, prevalence of asthma has decreased by 21% (95% CI : -30.1% to -10.6%) compared to before. Asthma hospitalization and ER visits also decreased by 10.8% (95% CI; -12.8% to -8.9%) and 17.1% (95% CI; -21.8% to -12.1%). The hazard ratio of 2012 births oh the incident asthma was significantly lower than births during HDs sale period (HR; 0.71, 95% CI; 0.69 to 0.73). Also we analyzed 4,733 children under 6 years with ILD. The incidence of J841, J848, and J849 have decreased by 43.2%, 81.2%, and 86% (95% CI : -90.4%, -79.7%). Conclusions: We concluded that the incidence of asthma and ILD in children have dramatically been reduced by policy implementation. However, it might be impossible to judge the independent policy effect of ban of HDs because various health-care polices have been implemented at the same time. It would be inherent limitation of epidemiological research. Therefore, further basic research will be essential to confirm the causal pathway between HDs and respiratory diseases in children.