Purpose: To evaluate (a) the association between Internet addiction and aggressive behaviors, as well as the moderating effects of gender, school, and depression on this association; and (b) to evaluate the association between Internet activities and aggressive behaviors. Methods: A total of 9405 adolescents were recruited into this study and completed the questionnaires. Their aggressive behaviors, with or without Internet addiction, Internet activities, demographic data, with or without depression, self-esteem, family function, and the watching of violent TV were assessed. Results: The results demonstrated that after controlling for the effects of shared associated factors and watching violent TV programs, adolescents with Internet addiction were more likely to have aggressive behaviors during the previous year. The association was more significant among adolescents in junior high schools than in senior high/vocational schools. Online chatting, adult sex Web viewing, online gaming, online gambling, and Bulletin Board System were all associated with aggressive behaviors. Conclusions: The results suggest that preventive programs for aggressive behaviors should pay attention to Internet addiction among adolescents. Also, intervention to prevent the effects of Internet addiction on aggressive behaviors should be conducted as early as possible. (C) 2009 Society for Adolescent Medicine. All rights reserved.