The aim of this study is to explore whether a participatory, organizational intervention can reduce work-related risk factors, and thereby prevent stress-related ill health. We build on the job demand-control and effort-reward imbalance models of stress. It is a two-armed randomized trial, with one primary health care unit receiving the intervention and a two-unit control group. Validated questionnaires for the assessment of psychosocial work environment and health were administered, at the baseline and at 6 and 12-month follow up. The primary outcome was job strain. Secondary outcomes were effort-reward imbalance, exhaustion, sleep, and recovery. Group-level objective data on workload and data about relevant processes during the study were continuously collected. The changes in the intervention group with regard to job strain, effort-reward imbalance, exhaustion, sleep and recovery were not statistically different from changes in the control group. For the non-exhausted employees though, reward was significantly higher at follow up compared to baseline, but only in the intervention group. An important piece of information is that the objective workload was statistically significantly higher in the intervention group throughout the study. Not all the components of the intervention were implemented as intended. Context and process information, such as objective data and implementation fidelity are necessary for a valid interpretation of the results.