OBJECTIVES: This study was done in adolescents with a high risk of substance or internet addiction in order to confirm the assumption that insecure attachment formation and impulsivity-inattention problems are major risk factors in adolescent addictive behavior. METHODS: 2,188 middle and high school students including a nicotine dependent treatment group were assessed using self-reporting scales : Adolescent Drinking Index (ADI), Fagerstrom Tolerance Questionnaire, Young Internet Addiction Scale, Revised Adult Attachment Scale (RAAS), and the Conner and Well's Self-reporting Scale for ADHD (CASS) were used. Subjects were classified into risk groups including substance addiction, internet addiction, as well as a combined group. RESULTS: Significant correlations were found between attachment formation and internet addiction tendencies, with respect to dependence, anxiety, and closeness (r=-0.185, r=0.248, r=-0.147, p<0.01, respectively). Impulsivity-inattention problems had positive correlations with internet addiction, alcohol addiction and nicotine addiction groups (r=0.345, r=0.211, r=0.187, p<0.01). With regard to attachment formation, the four groups showed significant differences regarding dependence (F=19.427, p<0.01), anxiety (F=28.926, p<0.01), and closeness (F=12.853, p<0.01). In addition, the four groups showed significant difference with respect to impulsivity-inattention problems (F=83.857, p<0.01), of which the combined risk group showed the highest scores, and the non-addicted group had the lowest scores. CONCLUSION: Insecure attachment formation and impulsivity-inattention problems were major risk factors for adolescent addictive behavior including internet addiction.