Objective: Determine the effectiveness of an educational intervention in order to increase the knowledge about Sexually Transmitted Infections (STI) in third-grade adolescents of "Los Jazmines de Naranjal" High School. Material and Methods: A pre-experimental, prospective and longitudinal study with a sample of 95 adolescents. At the beginning based on a Pre-test, an inadequate level of knowledge about Sexually Transmitted Infections was verified, that test was divided in four dimensions (definition, signs and symptoms, transmission and prevention). An educational intervention was implemented, with six sessions, where active learning strategies were used, and with a Post-test at the end of this. Descriptive analysis, frequency distribution and percentages, and the Wilconxon Test, were used to determine the intervention effectiveness. Results: In the sample, the 56% and 44% were female an...