Objective: To evaluate the knowledge of adolescents living in Vila Ana and Morada das Vinhas region, in the city of Jundiaí, State of São Paulo, Brazil, on prevention and diagnosis of the main sexually transmitted diseases (STDs) and on cervical cancer, as well as the immediate impact of educational lectures. Methods: A prospective cross-sectional study was performed to assess the knowledge of a particular group of female adolescents about STDs and cervical cancer, by means of a questionnaire applied before and after educational lectures. Results: After the lecture, there was an increased number of correct answers about sexual education, knowledge about HPV (44%), and prevention of cervical cancer (22%). Conclusion: The adolescents in our study had little knowledge about STDs and cervical cancer, but educative lectures could change this reality at a low cost to Public Health services.