INTRODUCTION: Adolescent health research can be hampered by a lack of representative participation, limited long-term engagement, and inaccuracies in self-reported responses. This study aimed to explore adolescents' perspectives on participating in health research in order to understand how to enhance engagement, representation, and data quality. METHODS: Forty-six adolescents (aged 16-18 years) at five English schools participated in nine focus groups held from March to July 2024. Constructivist grounded theory was deployed to explore factors influencing adolescents' participation in health research and the processes underpinning interactions between these factors. RESULTS: Participants raised five main considerations regarding adolescents' participation in health research. They believed that (1) positive relationships with researchers can foster an atmosphere of (2) emotional security that reduces the potential influence of (3) others' judgments on adolescents' relationships and emotional security. They felt that the more (4) choice and control adolescents have throughout the research cycle, the more likely they are to feel secure, which encourages them to remain engaged in research and provide accurate self-reported data. The combination of positive relationships with researchers and choice and control was thought to contribute to adolescents perceiving health research as an (5) influential collaboration, which empowers them to participate and provide accurate information, especially about their personal life experiences. CONCLUSION: This study suggests that enhancing adolescent engagement, representation, and data quality in research is possible but requires a substantial investment of time and resources. Such investment would enable researchers to foster positive relationships with adolescent participants by providing them with choice and control (where feasible) over research processes and by better communicating the potential impact of their participation. Given adolescent concerns about judgment and potential consequences of research participation, they are likely to benefit from clear, accurate, and reassuring conversations on these topics. PATIENT OR PUBLIC CONTRIBUTION: Two online meetings with a Young People's Advisory Group (YPAG) of adolescents aged 16 to 18 were conducted. In the first, eight participants provided feedback on a mock focus group's structure and content. In the second, they reviewed a conceptual model grounded in the focus group data, highlighting missing elements and aspects that lacked clarity.