Summary Mental health problems at work are currently a leading cause of work absences and have increased dramatically in recent years. Our previous research indicated that most workers absent because of a mental health problem mention difficulties in the workplace as a factor contributing to the deterioration of their health and their having to stop work—hence the importance of work-return practices that focus on modifying work organization factors. The general goal of this project is to track the move from an individual approach based in return-to-work support and employment retention for workers on leave due to a mental health problem to an organizational approach designed to prevent mental health problems in the workplace. This project is an extension of a study to design, implement and evaluate a return-to-work support initiative in a health and social services centre (St-Arnaud et al., 2011).Specifically, the project was meant to 1) identify and characterize work organization factors that the worker acknowledges as having contributed to the deterioration of his/her mental health and absence from work; 2) identify and characterize work organization factors that the worker acknowledges as a concern with respect to his/her return to work and has included in his/her return-to-work plan; 3) identify and characterize work-related organizational measures determined jointly by the worker and his/her supervisor and included in the return-to-work plan to support the worker when he/she returns to work (tertiary prevention); 4) determine discrepancies between factors identified by the worker, measures included in the action plan and measure actually implemented by the organization, and analyze what accounts for these discrepancies and might impact primary prevention intervention; and 5) identify primary prevention targets, based on the entire process undergone. The study focuses specifically on analyzing of the process undergone by people on mental health leave who have identified work as a factor that contributed to the deterioration of their health and their absence from work. Data were collected through semi-structured individual interviews of workers and their immediate supervisors, and from action plans written jointly by the worker and the employer to support the return to work. The software NVivo was used to code and analyze the data. This research project made it possible to develop a return-to-work support strategy based on work activities and on implementation of tertiary prevention measures based on real situations and in cooperation with the immediate supervisor. This strategy also led to discovery of possible primary prevention interventions for mental health in the workplace, interventions that might help workers who are not yet on mental health leave but might be affected by work situations that caused other workers to take such leave. What is original about this research project is the sentinel approach to the development of preventive practices in workplace mental health. This approach calls for study of cases of workers on mental health leave who pinpoint work organization elements as contributing to their pathology in order to identify work situations that might also affect other workers in the same work environment.