Background Prospective studies on the relationship between organisational merger and mental health have been conducted using subjective health indicators. The objective of this prospective occupational cohort study was to examine whether a negative change during an organisational merger is an independent predictive factor of psychiatric morbidity.

Method Survey data on organisational characteristics, health and other factors were collected prior to (1996) and after the merger (2000); register data on psychiatric morbidity were collected at baseline (1/1/1994–30/9/2000) and during the follow-up (1/10/2000–31/12/2005). Participants were 6511 (77% men) industrial employees aged 21–65 years with no register-based diagnosed psychiatric events prior to the follow-up (the Still Working Study). During the follow-up, 252 participants were admitted to the hospital due to psychiatric disorders, were prescribed a psychotropic drug or attempted or committed suicide.

Results A negative self-reported change in the work organisation during the merger was associated with increased risk of postmerger psychiatric event (HR 1.60, 95% CI 1.19 to 2.14). This association was independent of mental health-related factors measured before the merger announcement, such as demographic characteristics, occupational status, personal orientation to life, self-rated health, self-reported psychiatric morbidity or chronic disease.

Conclusion A negative change in work organisation during an organisational merger may elevate the risk for postmerger psychiatric morbidity.