This study aimed to analyze the level of knowledge concerning emergency contraception among adolescents in public and private high schools. This was a cross-sectional study with 705 students 15 to 19 years of age enrolled in public and private high schools in a municipality in Sao Paulo State, Brazil. The authors used stratified probabilistic sampling by type of school and systematic sampling by class. Sexual initiation and use of emergency contraception were reported by 24.9% of private school students and 32% of public school students. The mean score on knowledge was 3.87 (SD = 2.12) in public schools and 5.14 (SD = 2.00) in private schools. Multiple linear regression analysis showed that higher scores on knowledge concerning emergency contraception were associated with: enrollment in private schools, female gender, older adolescents, sexual initiation, previous use of emergency contraception, and knowing someone who had used the method. The study concludes that few adolescents are properly informed about the method and that many harbor persistent misconceptions.