The incidence of infection with HIV/AIDS among the heterosexual population has been increasing in young adults. The goal of this research was to deepen knowledge of preventive sexual behavior in Portuguese adolescents, including knowledge and attitudes about HIV/AIDS, and assessing whether they changed from 2002 to 2006. Data were collected through a self-administered questionnaire from the Portuguese sample of the Health Behavior in School-aged Children (HBSC), a collaborative WHO study. The study provides national representative data of 7093 Portuguese adolescents, randomly chosen from those attending 8th and 10th grade of high school. Results showed there was an increase in the age of first sexual intercourse and a decrease in the number of teenagers who reported having had sexual intercourse, also in the level of information regarding HIV/AIDS transmission/prevention and in positive attitudes towards people with HIV/AIDS. In general, adolescents have good knowledge about how to protect themselves from becoming infected. However, comparing to 2002, there was a reduction of knowledge and consequent increase in the doubts regarding HIV/AIDS. Given the incipient state of sex education in 2006, the results cannot be attributed to sex education, but they will be relevant for comparison with the 2010 HBSC results.