Prospective research on psychosocial effects on employees' health associated with organizational mergers has been scarce. The first aim of this study was to explore the subjective health effects (exhaustion and functional incapacity) of an organizational merger among employees who had experienced a change in their own job position differently (improved, unaltered, and declined). Secondly, the effects of pre-merger social support (organizational, supervisor, and coworkers) at work on the experienced change in job position and on subjective health were examined. The merger took place in 1999 between two multinational firms of equal size. The study is based on two surveys (n = 2,225) carried out in 1996 and in 2000 in the Finnish part of the company. The data on age, sex, pre-merger sickness absence (1996-98) and subjective health status (1996) were used as covariates. The results indicate that all sources of social support had a significant effect on the experience of change in one's job position. A decline in job position strongly increased the risk of poor subjective health after the merger. Weak organizational support was associated with impaired subjective health, especially in blue-collar workers, while weak supervisor support impaired functional capacity in white-collar workers, In turn, strong co-workers' support increased the risk of poor subjective health among blue-collar workers when their job position declined. We conclude that negative changes experienced in one's job position and lack of upper-level social support at work create a potential risk for health impairment in different employee groups in merging enterprises. (C) 2003 Elsevier Ltd. All rights reserved.