Objectives. To develop and validate an instrument to assess Internet Addiction (IA) phenomenon in adolescents of Metropolitan Lima. Materials and methods. We performed an observational analytical study, including a sample of 248 high school adolescent students. In order to evaluate the IA, we constructed the questionnaire: "Scale for Internet Addiction of Lima" (SIAL), which assesses symptoms and dysfunctional characteristics. The resulting items were submitted to experts' judgment, finally obtaining a 11-item scale. Results. The mean age was 14 years old. The psychometric analysis of the instrument showed a Cronbach' Alpha Coefficient of 0.84, with values of item-total correlation ranging from 0.45 to 0.59. The dimensional analysis yielded a two-dimensional structure that explained up to 50.7% of the total variance. The bi-dimensional data analysis revealed a significant association (p<0,001) between Dimension I (symptoms of IA) and the weekly time spent on the Internet, male sex, past history of bad behavior in school and plans for the future. Dimension II (dysfunction due to IA) had a significant association to past history of bad behavior, plans for the future (p<0,001) and missing school without valid reasons. Conclusions. The SIAL showed a good internal consistency, with moderate and significant inter-item correlations. The findings show that addiction has a dynamic role, which evidences a problem generated in family patterns and inadequate social networks.