Tehran University of Medical Sciences Iranian Journal of Public Health 2251-6085 49 11 2020 10 27 2069 2077 Yuichiro OTSUKA Division of Public Health, Department of Social Medicine, Nihon University School of Medicine, Tokyo, Japan Yoshitaka KANEITA Division of Public Health, Department of Social Medicine, Nihon University School of Medicine, Tokyo, Japan Osamu ITANI Division of Public Health, Department of Social Medicine, Nihon University School of Medicine, Tokyo, Japan Mikiko TOKIYA Department of Public Health and Epidemiology, Faculty of Medicine, Oita University, Oita, Japan 2018 09 12 2019 03 17 Background: Internet addiction and poor mental health are two pervasive problems during adolescence. This study aimed to determine whether Internet addiction and poor mental health status exhibited a bidirectional relationship in which either variable could become a risk factor for the onset of the other. Methods: Longitudinal school-based survey with a baseline sample of 1547 students among 8 schools (10th graders) in Japan surveyed in 2015 and followed up 1 year later. The schools are located in the middle city. We assessed internet addiction using the Japanese version of the Diagnostic Questionnaire developed by Young and mental health status using the 12-item General Health Questionnaire. Regression analyses including the covariates tested whether Internet addiction was related to the onset of poor mental health among youth who had never internet addiction, and poor mental health was related to the onset of internet addiction. Results: The incidence for Internet addiction and poor mental health during one year were 22.0% and 8.8%, respectively. Multiple regression analyses showed that poor mental health (adjusted odds ratio: 2.17 [95%CI: 1.45–3.25]) promoted new onset of Internet addiction and Internet addiction (adjusted odds ratio: 2.39 [95%CI: 1.36–4.20]) also promoted new onset of poor mental health. Conclusion: Internet addiction and poor mental health status each increased the risk of onset of the other. Adolescents, their parents and schools need to take policies to use properly Internet. https://ijph.tums.ac.ir/index.php/ijph/article/view/14702 https://ijph.tums.ac.ir/index.php/ijph/article/download/14702/7033