Objective: The STI/HIV prevention programme in Amsterdam aims to improve the sexual health of Amsterdam residents of African, Antillean, Aruban and Surinamese origins. The programme strategy is to achieve and enhance the participation of migrant community-based organisations (CBOs) in sexual health promotion through a grant scheme and by providing support to CBOs in developing, implementing and monitoring STI/HIV projects within their own communities. This evaluation focussed on identifying obstacles and successes of the CBO projects, assessing the organisation of the programme and examining the level of CBO participation. Design: Multi-method qualitative approach based on a community-based research framework, including document analysis, 37 semi-structured interviews with CBO representatives and Public Health Service (PHS) professionals and participant observation. Setting: Amsterdam, The Netherlands. Results: In total, 76 CBOs participated in the programme, and 110 diverse and culturally specific projects were developed. The CBOs appreciated participation in the programme, but did not necessarily perceive their participation to be at a high level. The multiple roles fulfilled by the PHS coordinator and the capacity of both the PHS and the CBOs were crucial for success. Conclusion: This study shows that the strategy to achieve and enhance participation by CBOs in combination with grant schemes is efficient for developing and implementing STI/HIV projects, provided that there is on-going support to develop, implement and monitor projects. In assessing the effectiveness of the programme, a number of challenges lie ahead, especially regarding the continuous monitoring of projects and assessing programme impact on sexual health.