This article describes the development and evaluation of an HIV prevention program developed for early adolescents through participatory action research. The HIV prevention program included a curriculum that was delivered by trained younger youth leaders through a youth and adult partnership with 10 schools in Chiang Mai Province, Thailand. The curriculum used participatory learning experiences, "edutainment" approaches, and skills-building strategies for enhancing youth leaders' capacities. Results of the evaluation showed that the senior-junior peer education program was effective in leadership role preparation, in improving youth leaders' ability to share sexual and reproductive health knowledge, and in promoting positive attitudes toward themselves. Success also rested on the fact that adults took a critical role in providing the opportunities, assistance, and guidance so that young people could develop their leadership capacity in an atmosphere of trust and respect.