In most emergency departments (EDs), few patients account for a relatively high number of ED visits. To improve the management of these patients, the university hospital of Lausanne, Switzerland, implemented an interdisciplinary case management (CM) intervention. This study examined whether the CM intervention-compared with standard care (SC) in the ED-reduced costs generated by frequent ED users, not only from the hospital perspective, but also from the third-party payer perspective, that is, from a broader perspective that takes into account the costs of health care services used outside the hospital offering the intervention. In this randomized controlled trial, 250 frequent ED users (>5 visits during the previous 12 months) were allocated to either the CM or the SC group and followed up for 12 months. Cost data were obtained from the hospital's analytical accounting system for the entire sample and from health insurance companies for a subgroup (n = 140). Descriptive statistics and multivariate regressions were used to make comparisons between groups and assess the contribution of patient characteristics to the main cost components. At the end of the 12-month follow-up, 115 patients were in the CM group and 115 in the SC group (20 had died). Despite differences in economic costs between patients in the CM intervention and the SC groups, our results do not show any statistically significant reduction in costs associated with the intervention, either for the hospital that housed the intervention or for the third-party payer. Frequent ED users were big users of health services provided by both the hospital and community-based services, with 40% of costs generated outside the hospital that housed the intervention. Higher age, Swiss citizenship, and having social difficulty increased costs significantly. As the role of the CM team is to guide patients through the entire care process, the intervention location is not limited to the hospital but often extends into the community.