The US healthcare system is the most costly in the world, yet does not produce superior results of healthcare quality. Moreover, costs of care are not equally distributed in the population – instead a small proportion of patients are responsible for a high proportion of cost in the US healthcare system.  For instance, 1% of patients are responsible for approximately 20% of healthcare costs and these patients typically have multiple chronic conditions as well as mental health problems.  In this paper we evaluate the impact of Care One, an intensive case management and care coordination services that is provided to the most costly (top 1%), medically complex emergency department patients in Albuquerque, New Mexico.  Specifically, a parametric, difference-in-difference model is estimated, evaluating the impact of receiving Care One treatment on hospital billing charges.  Preliminarily results find that the Care One cohort had greater reductions in billing charges than a constructed quasi-control group, suggesting that intensive case management may be an effective mechanism to more effectively manage medically complex patients and contain costs in Emergency Departments.