Objective: Sex education is essential for an accurate approach of sexuality in adolescents, being more effective when it is carried out at early ages. This study aimed to estimate the sexual health knowledge and the information in this regard perceived by adolescents, as well as where it came from., Methods: A cross-sectional observational study was performed in students from two public high schools in Castilla-La Mancha who were surveyed about sexual knowledge. Comparisons were made by using the Student's t test or Mann Whitney U test, the Chi-square test (or Fisher's exact test) depending on the variables., Results: 248 students were included (68.1% from Madridejos and 31.9% from Herencia) with a mean age of 14.67±1.1 years. The 47.2% were women. Only 4.8% of the participants affirm to talk about sexuality with their parents. The 56% declared had consumed pornographic content, with a mean first access age of 12.8±1 years. Significant statistically differences were found between the information received in schools and in the familiar surroundings, being alcohol and drug intake, sexually transmitted infections (STI), and contraceptive methods more frequently treated at the educational centers. A 25% of the surveyed students who reported having performed sexual relations stated not having used condom. Approximately, half of the participants did not perceive a high risk of getting STIs (HIV, herpes and chlamydia) in the case of having sex without condom., Conclusions: There is a lack of sexual knowledge in adolescents, perceiving a noteworthy heterogeneity in the offered information depending on it is received in high school or in familiar surroundings.