Health promotion interventions with marginalised groups are increasingly expected to demonstrate genuine community participation in their design and delivery. However, ideals of egalitarian democratic participation are far removed from the starting point of the hierarchical and exploitative social relations that typically characterise marginalised communities. What scope is there for health promotion projects to implement ideals of community leadership within the realities of marginalisation and inequality? We examine how the Sonagachi Project, a successful sex-worker-led HIV prevention project in India, has engaged with the unequal social relations in which it is embedded. Our ethnographic study is based on observation of the Project's participatory activities and 39 interviews with a range of its stakeholders (including sex worker employees of the Project, non-sex-worker development professionals, brothel managers, sex workers' clients). The analysis shows that the project is deeply shaped by its relationships with non-sex-worker interest groups. In order to be permitted access to the red light district, it has had to accommodate the interests of local men's clubs and brothel managers. The economic and organisational capacity to run such a project has depended upon the direct input of development professionals and funding agencies. Thus, the 'community' that leads this project is much wider than a local grouping of marginalised sex workers. We argue that, given existing power relations, the engagement with other interest groups was necessary to the project's success. Moreover, as the project has developed, sex workers' interests and leadership have gained increasing prominence. We suggest that existing optimistic expectations of participation inhibit acknowledgement of the troubling work of balancing power relations. Rather than denying such power relations, projects should be expected to plan for them.