Drawing on a case study, this article questions the role of planning and management strategies in the process of transforming a regional public healthcare system that involves a number of organizations and is characterized by fluidity in its functions and division of power. It examines the efficacy of the Regional Plan for the Organization of Health Services (PROS) in reforming the mental health sector in a health and social service district in Quebec, in terms of integrated regional management of mental healthcare and activities at the local level. The regional planning procedure involves a major transformation in management of the mental health system, organizational roles and clinical and professional practices. Our assessment of PROS highlights the importance of taking into account the context of implementation and the instrumental value of planning, before judging its efficacy. To transform a complex healthcare system at the regional and local level, the study suggests a revised conception of the main roles played by planning and of the process shaping its implementation. Our study concludes in favour of developing management strategies at the operational, clinical and professional levels, and integrating them as a planning aid that allows a more corporate and matrix-based system to be set up.