Recent literature draws attention to the fact that problematic Internet use is a growing health issue among adolescents worldwide. This study aimed to find the potential demographic, physical and psychopathological risk factors associated with severe problematic Internet use of adolescents. A total of 1,347 adolescents (45.1% males) were recruited from five high schools in three settlements. Students completed an online questionnaire and provided data about socio-demographics, physical activity, physical and psychopathological health, as well as about their Internet use habits (Problematic Internet Use Questionnaire). 1.9% of the respondents appeared to be severe, 20.4% moderate problematic Internet users. The complexity of the problem is manifested in the multifaceted relationships of factors. The multinomial regression analysis identified four significant risk factors for severe problematic internet use, namely depressive symptoms, loneliness, low level of father's education level and low level of physical activity. The real severe problematic Internet use is not so common in adolescent students, however, together with moderate problematic Internet users, this number has significance. Psychopathological factors such as elevated depressive symptoms and loneliness should be considered when preventive programs are implemented and more attention should be taken on physical activity as an effective tool against addictive behavior.