Background: The internet has become an important tool for social interaction, information, and entertainment; particularly among adolescents. Aim of the study : was to assess determinants of problematic internet use among secondary school students at Zagazig City. Subjects & Methods: Research design: A cross-sectional descriptive design was utilized. Setting: four secondary schools (El-Zagazig for girls, Sheba, Zagazig Hotellery, and New Commerce) affiliated to Zagazig City. Subjects: a sample of 500 randomly selected students Tools of Data collection: Tool (1): Questionnaire composed of three parts; general characteristics, questions about internet using and Young Internet addiction test (YIAT). Tool (2): Strengths and Difficulties Questionnaire (SDQ). Tool (3): Personality test. Results: The present study revealed that the prevalence of problematic internet use (PIU) and potential PIU were 5% and 95% respectively among the studied subjects. Poor family relations, accessibility of internet, mobiles, and prolonged periods of using internet were the most contributing risk factors of PIU. Conclusion: PIU is not rare among secondary school students and there were many adverse consequences of prolonged use of internet as poor academic achievement. Recommendations: Strengthen preventive efforts for reducing PIU and related consequences among adolescents through giving health education in schools about disadvantages of prolong duration of internet use.