While adolescents are increasingly using the Internet for health information, little research has been done to assess and improve their "eHealth literacy"-the abilities to find, evaluate, and apply online health information. This study examines the extent to which adolescents' levels of eHealth literacy can be improved by known determinants such as social cognitive factors and perceived social influences, either independently or jointly. Among 182 middle-schoolers, an eHealth literacy intervention was carried out. It involved qualitative and quantitative baseline research, three online training sessions, and a postintervention survey. According to hierarchical regression model results, social cognitive factors of outcome expectations and involvement, but not health motivation, significantly improved eHealth literacy, and all the perceived social influence variables significantly improved eHealth literacy. However, no joint effect of social cognitive factors and perceived social influences was found. In light of these findings, educators need to make eHealth literacy programs personally relevant to adolescents and reinforce local social norms about the importance of seeking health information online.