Purpose: Adolescents are well served by the Internet through school access, but they will only derive maximum benefit from this information resource if they are able to search for, evaluate, and use its online information effectively. Internet use, however, requires significant literacy skills, and little is known about the health literacy of adolescents. The aim of this paper is to describe functional, critical, and interactive health literacy challenges experienced by adolescent students when using the Internet for online health information. Methods: Twenty-six focus groups with 157 adolescent students, aged 11-19, conducted within a convenience sample of middle and high schools in diverse geographical and socioeconomic settings in the United Kingdom (UK) and United States of America (US) between May 2001 and May 2002. Results: Many students cited difficulties in accessing health information online. Functional health literacy challenges included, for example, spelling medical terms correctly and being able to construct questions describing symptoms accurately. Critical challenges included discerning relevance of information retrieved by search engines and knowing which sites to trust. Interactive challenges included the appropriate application of health information to address personal health concerns within their local neighborhood. Conclusions: Exploring the challenges faced when adolescents search for online health information indicates deficiencies regarding health literacy skills. Difficulties regarding functional, critical, and interactive skills were all present. The Internet may offer opportunities for identifying such deficiencies and building better health literacy skills among adolescents, as part of health curriculum interventions to improve the population's future health.