Purpose -Knowledge loss caused by employee exit has become a significant corporate risk. This paper aims to explore how to measure the impact of knowledge loss. The paper is based on empirical evidence from a five-year longitudinal study. Design/methodology/approach -This paper is based on a longitudinal change project for a large Australian Research Council Linkage Project grant in the period 2008-2013. The method was a single case study using a critical realism paradigm. The project was a transformational change programme which aimed to help make the partner organization a learning organization to minimize the impact of knowledge loss. The partner organization was a large Australian Government Department, which faced the threat of knowledge loss caused by its ageing workforce. The sample was 118 respondents, mainly engineering and technical workers. A total of 150 respondents were invited to participate in the study which involved an annual survey and attendance at regular training workshops and related activities, with a participation rate of 79 per cent. Findings -The results found that knowledge loss has most negative impact in terms of organizational problems including low productivity (morale), strategic misalignment of the workforce (capability gaps), resource cuts (stakeholders unhappy with performance), decreased work quantity and quality (inexperienced employees), work outputs not being used (customers mistrust), longer time to competence (learning cost) and slow task completion (increased search cycle time). The second most significant impact was increased sense of risk associated with work activities and declining capacity to manage the risk. The third main impact was decreased organizational knowledge base: knowledge loss creates knowledge deficit which is unlikely to be filled over time, as shown by the knowledge accounts of surviving employees which remained stable overall. The two remaining measurement constructspsychological contract and learning organizational capacityimproved, which suggests that the negative impact of knowledge loss may be addressed with appropriate knowledge management.Research limitations/implications -The research is based on a single case study in a public sector organization. While the longitudinal nature of the study and the rich data collected offsets this issue, it also presents good opportunities for researchers and practitioners to test the ideas presented in this paper in other industry contexts. The complexity and range of the constructs, concepts and scale items is acknowledged. Tables have been used wherever possible to help the reader access the findings. Practical implications -Knowledge loss is perhaps the greatest corporate risk facing organizations today. This paper provides a method to measure the impact of knowledge loss. Managers may use this to assess the significance of the risk and use this as a business case to take action to minimize the impact of knowledge loss.Originality/value -Prior research has found knowledge loss has caused decreased psycholo...