Background: Little research has examined adolescent health literacy and its relationship with online health information sources. The purpose of this study is to explore health literacy among a predominantly Hispanic adolescent population and to investigate whether exposure to a credible source of online health information, MedlinePlus[R], is associated with higher levels of health literacy. Methods: An online survey was administered to a cross-sectional random sample of high school students in South Texas. Self-reported sociodemographic characteristics and data on health-information-seeking behavior and exposure to MedlinePlus[R] were collected. Health literacy was assessed by eHEALS and the Newest Vital Sign (NVS). Linear and binary logistic regressions were completed. Results: Of the 261 students who completed the survey, 56% had heard of MedlinePlus[R], 52% had adequate levels of health literacy as measured by NVS, and the mean eHEALS score was 30.6 (possible range 8-40). Health literacy was positively associated with self-efficacy and seeking health information online. Exposure to MedlinePlus[R] was associated with higher eHealth literacy scores (p less than 0.001) and increased the likelihood of having adequate health literacy (odds ratio: 2.1; 95% CI: 1.1, 4.1). Conclusion: Exposure to a credible source of online health information is associated with higher levels of health literacy. The incorporation of a credible online health information resource into school health education curricula is a promising approach for promoting health literacy. (Contains 4 tables.)