Objective: To establish the sexual practices and the associated factors of the sexual behavior of school adolescents of Cartagena. Methodology: Transversal study, population 104.088 adolescent students of Secondary School of Public and private schools of Cartagena. Sample 1.035 adolescents, confidence level 95 %, and error 2 %. Sampling: multistage. Two scales of knowledge and attitudes were used, and the Francis scale. Results: The average age to start sexual relations was 14.1, 17% of the adolescents have had sexual relationships. To have sexual relations is associated significantly with the fact of being male, being in a higher grade, cigarette consumption, belonging to a social group, gender inequality, having a partner and considering him/her stable. Sexual relations are more common in public schools than in private schools. Conclusion: Out of 22 factors studied, eight factors were strongly associated with the start of sexual relations.