Objective This study assessed the long-term effects of a workplace intervention aimed at reducing adverse psychosocial work factors (psychological demands, decision latitude, social support and effort–reward imbalance) and mental health problems among health care professionals in an acute care hospital.

Methods A quasi-experimental design with a control group was used. Pre-intervention (71% response rate) and 3-year post-intervention measures (60% response rate) were collected by telephone interviews with validated instruments.

Results Three years after the intervention, all adverse psychosocial factors except one were reduced in the experimental group, and the improvement was statistically significant for 5/9 factors: psychological demands, effort–reward imbalance, quality of work, physical load and emotional demands. In addition, all health indicators improved and 2/5 significantly: work-related and personal burnout. In the control hospital, three work factors improved significantly but two deteriorated significantly: decision latitude and social support. All health problem deteriorated, although not significantly, in the control hospital. Moreover, 3 years after the intervention, the mean of all adverse factors except one (psychological demands) and all health indicators was significantly more favourable in the experimental than the control hospital, after adjusting for pre-intervention measures.

Conclusion These results support the long-term effectiveness of the intervention. The reduction in many psychosocial factors in the experimental hospital may have clinical significance since most health indicators also improved in this hospital. These results support the whole process of the intervention given that significant improvements in psychosocial factors and health problems were observed in the experimental hospital but not in the control hospital.