OBJECTIVES. Few data have been available among adolescents to determine behavioral changes that may prevent human immunodeficiency virus (HIV) infection. This analysis examines changes in the prevalence of self-reported HIV-related information sources, instruction, knowledge, and behaviors among high school students in the United States. METHODS. Two independent, multistage national probability samples of students in grades 9 through 12 were surveyed in 1989 (n = 8098) and 1990 (n = 11,631) with self-administered, anonymous questionnaires that included similar items. RESULTS. Compared with students surveyed in 1989, a significantly greater proportion of students surveyed in 1990 had received HIV instruction in school. Significant decreases were found in the proportion of White and female students who reported having had sexual intercourse, in the proportion of White students reporting two or more lifetime sex partners, and in the proportion of 15- and 16-year-olds, White students, and female students who reported having had four or more lifetime sex partners. For both years, students who had a greater level of HIV knowledge were less likely to have had multiple lifetime sex partners or to have injected illicit drugs. CONCLUSIONS. The findings suggest that school-based HIV education and knowledge may be contributing factors in reducing certain risk behaviors that can lead to HIV transmission among secondary school youth.