Background: Previous reports show that Chilean teenagers have an inadequate knowledge about sexuality and reproduction. Aim: To compare the knowlege about sexuality among adolescents coming from private and public schools, with and without sexual education programs. Material and methods: A structured anonymous inquiry, containing multiple choice and open questions, was applied to a sample of 229 adolescents attending seventh and eigth grade of junior school, in private and public schools of Temuco, Chile. Results: Eleven percent of adolescents had already their first sexual intercourse at a mean age of 12.2 ± 2.4 years old. Of these, 96% came from public schools. An overall analysis of tests, disclosed a 53% of correct answers to the inquiry. Adolescents coming from private schools had a better performance than those coming from public schools. Sexual attitudes were not influenced by sexual education programs. Conclusions: Adolescents coming from private schools have a better sexual knowledge level and more conservative attitudes towards sexuality. Overall knowledge is inadequate albeit overvalued. These teenagers are high risk group for unwanted pregnancies and sexually transmitted diseases and require efficient sexual education programs. (Rev Méd Chile 2000; 128: 574-83).