In 2004, several environmental health investigations were marked by strained relations with local communities because they only partially addressed broader social concerns and multiple issues that were difficult to grasp. Drawing on two investigations conducted in 2006, this article explores the benefits of establishing an early and dynamic relationship with local communities during the health impact assessment process. The assessment of the various issues, developed through dialogue with local communities, is a necessary prerequisite for determining the extent to which the expected response requires the expertise of the French Institute for Public Health Surveillance (InVS), for clarifying what the Institute is capable of doing and what it is not, and ultimately for proposing an appropriate investigation. Furthermore, community participation in the investigation enriches the protocol with local specifics and contributes to better study conduct. Finally, discussions regarding the results can facilitate the integration of recommendations by the community that will be directly affected. This consultation can only improve the quality of decision support. With the help of social science professionals, the InVS has developed two tools presented in this article. The next step would be to share, with risk managers, reflections on the shared benefits of broader stakeholder involvement. (R.A.)