The objective of the article is to present a destructive impact of mergers and acquisitions on employees, which is considered to be a key barrier in achieving their expected performance. Analyses are based on theoretical and empirical research conducted by the author while completing her doctoral dissertation on human resource management in mergers and acquisitions. Firstly, individual perceptions and key factors of stress are discussed. Secondly, merger syndrome is presented as well as an impact of employees' behavior on merger performance. Finally, effective methods of reduction of a negative impact of mergers and acquisitions on employees are proposed.