An exploratory, descriptive, comparative study with a quantitative approach, whose objective was to compare the knowledge of high school students from two public state schools of Peruibe, SP, Brazil, about Human Immunodeficiency Virus (HIV) and Acquired Immunodeficiency Syndrome (AIDS), in the years 1999 and 2010. In both of these years, the predominant population studied was female students, under 18 years old, unmarried and white. A statistically significant difference was found with respect to knowledge about HIV/AIDS among the groups studied. It was found that even with ten-year interval between the studies, the two groups of students did not consider themselves vulnerable to HIV and presented doubts as to the correct knowledge about the topic, which indicates the need for a continuous approach with young adolescents.