Formal sexual education is a mandatory component of the high school curriculum in most Canadian provinces. The present study was a preliminary assessment of sexual knowledge among a sample of Ontario adolescents who had completed their high school sexual education requirements. A questionnaire, testing understanding of the learning objectives of Ontario's minimally required high school sexual education course, was distributed in a paediatric emergency department to 200 adolescent patients who had completed the course. Respondents demonstrated good understanding of pregnancy physiology and sexually transmitted infections, but poor understanding of concepts related to reproductive physiology, contraception, HIV/AIDS and sexual assault. Most respondents could not identify Canada's age of sexual consent. Respondents demonstrated concerning gaps in sexual knowledge despite completion of their sexual education requirements. Further studies must determine whether a representative, population-based student sample would exhibit similar findings. Sexual education currently offered in Ontario may require investigation.