Background: Prospective studies on the relationship between organizational 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 organizational merger is an independent predictive factor of psychiatric morbidity. Method: Survey data on organizational characteristics, health, and other factors were collected prior to (1996) and after the merger (2000); register data on psychiatric morbidity at baseline (1/1994--9/2000) and during the follow-up (10/2000--12/2005). Participants were 6 511 (77% men) industrial employees aged 21--65 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 hospital due to psychiatric disorders, were prescribed a psychotropic drug, or attempted or committed suicide. Results: A negative self-reported change in the work organization during the merger was associated with increased risk of post-merger psychiatric event (Hazard Ratio 1.60; 95% Confidence Interval 1.19-2.14). 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. Conclusions: A negative change in work organization during an organizational merger may elevate the risk for post-merger psychiatric morbidity.