The aim of this study was to investigate the vulnerability of adolescent students related to sexually transmitted diseases (STD) and the human immunodeficiency virus (HIV), identifying the main risk behaviors and prevention. This quantitative, cross-sectional study was performed in three public schools in Imperatriz--Maranhão, with 295 adolescents, using a structured questionnaire. The results show that: most young people (86.3%) who used a condom the last time they had intercourse, usually keep this practice, 82.8% of adolescents who understand the concept of HIV protect themselves against these infections and believe the main form of contamination is through sex, infected blood or through the placental barrier. We conclude that most teenager participants showed coherent knowledge about sexual practices and risk behaviors that make them vulnerable to STD/HIK presenting a positive aspect for the prevention of these diseases.