Community-researcher partnerships can be powerful mechanisms to understand and effectively address health and social problems such as HIV/AIDS prevention. When the partnership is a positive, productive one, the combined expertise and energy of both parties result in a more effective program and a better evaluation of its effects. This article describes one such partnership and how a program challenge provided the opportunity for both partners to develop new capacities and strengthen others. The program is Proyecto SOLAAR, a community-based and culturally-sensitive HIV prevention program for gay and bisexual Latino men. The program is an experiential, daylong retreat focused on personal aspects of the men (e.g., self-concept), ideas about and aspects of their relationship behavior (e.g., cultural misunderstandings, dating behavior), and HIV prevention; there is a follow-up reunion a month later to share experiences with other participants about new dating and HIV prevention behaviors. The article focuses in particular on how the partners built new capacity in the area of social marketing to address the challenge of participant recruitment and describes the components of the new campaign. These components included distinctive images in ads in publications read by the target population, a toll-free telephone number and Web site for easy initial contact with the program, phone cards and postcards featuring the specially created program image to reinforce a connection to the program, and other aspects. The article describes the partnership between the HIV service providers and the researchers and how the collaborative effort was key to understanding and addressing the recruitment problem, identifying potential solutions, and implementing the new social marketing strategy. This process resulted in four kinds of capacities that were built or strengthened, including program recruitment, program content and implementation, program evaluation, and the partnership itself. The article concludes with a discussion of the unexpected benefits of the capacity-building experience and of the antecedent conditions that fostered the positive partnership outcomes.