There is a recent decline in HIV in a significant number of countries due to the adoption of preventive sexual behaviors, which demonstrates that HIV reduction is possible. The goal of this research was to deepen knowledge of preventive sexual behavior in adolescents, including knowledge and attitudes about HIV/AIDS, and assessing whether they changed from 2002 to 2010. Data were collected through a self-administered questionnaire from the Portuguese sample of the Health Behavior in School-aged Children, a collaborative WHO study. The study provided national representative data of 10,587 Portuguese adolescents attending 8th and 10th grade. In terms of preventive behaviors, results showed an increasing trend regarding the percentage reporting first sexual intercourse at 14 years old or more and condom use at last intercourse and a stabilized trend concerning having had intercourse, contraceptive pill use at last intercourse and having had intercourse under the influence of alcohol or drugs. Nevertheless, results showed a systematic decreasing trend in terms of knowledge and attitudes. This suggested that sex education programs are still too limited to teaching sessions, strongly homogenized in their content and inadequate to enhance knowledge and attitudes regarding HIV, let alone personal and social skills of different target groups.