This paper examines how Vietnamese adolescents’ access to health information, especially concerning HIV, and their level of social capital might predict their differential levels of HIV knowledge. It uses data from the 2003 Vietnam Survey Assessment of Vietnamese Youth (VNSAVY), which collected information on Vietnamese adolescents’ level of HIV knowledge, access to health information, and their social capital. The survey is the first population-based survey on Vietnamese young people and involved 7584 adolescents and young adults. This study employs k-means clustering technique with cross-validation procedure to build an indicator of social capital. Analyses involve weighted generalized ordinal regression, finding that having access to more information sources increases the level of accurate HIV knowledge among Vietnamese adolescents, though the effect is strongest among those with the lowest level of HIV knowledge and much weaker among adolescents already having an average level of HIV knowledge. Having a cohesive family, a strong social network, and peers who encourage healthy behaviors and discussions of sexual heath help adolescents achieve a higher level of HIV knowledge. The importance of positive social capital also has differential affects across HIV knowledge levels. The authors conclude that having more access to health information and more positive social capital increases HIV knowledge among Vietnamese adolescents, but the effect of these factors differs across groups. The findings suggest that policies aimed at improving adolescents’ level of knowledge need to diversify and should include programs that enhance social capital for adolescents, especially in ways that might encourage frank discussion regarding sexual behaviors. Such efforts could include peer counseling, family education, and community communication enrichment.