Since 1st January 2011, compulsory military service was abolished and transition was made to manning the units of the Serbian Armed Forces with professional members. This implies that the Serbian Armed Forces should attract, employ and retain professional soldiers within their units. Such scenario has created new challenges in human resource management within the Serbian Armed Forces. Having in mind that professional soldiers make a significant component that influences the state and operational capabilities of Serbian Armed Forces units, turnover represents a serious obstacle both in achieving and maintaining required level of their operational capability. Every organisation faces turnover. It is necessary for the management of the defence system to identify expectations and requirements of professional soldiers, which would contribute to the development of a strategic plan for the reduction of their turnover. The goal of this paper is to identify the causes of the turnover of professional soldiers of the Serbian Armed Forces. Empirical research was carried out on a sample of 117 company/autonomous platoon commanders and 1213 professional soldiers of the Serbian Armed Forces. The causes of turnover were examined through a survey. Company/ autonomous platoon commanders were asked to evaluate factors that encourage professional soldiers the most to leave the Serbian Armed Forces. Professional soldiers were asked to define actions that the defence system should take to reduce their outflow. Obtained results show that the turnover of professional soldiers is considerably influenced by job satisfaction and organisational support.