The Medical Entities Dictionary (MED) has served as a unified terminology at New York Presbyterian Hospital and Columbia University for more than 20 years. It was initially created to allow the clinical data from the disparate information systems (e.g., radiology, pharmacy, and multiple laboratories, etc.) to be uniquely codified for storage in a single data repository, and functions as a real time terminology server for clinical applications and decision support tools. Being conceived as a knowledge base, the MED incorporates relationships among local terms, between local terms and external standards, and additional knowledge about terms in a semantic network structure. Over the past two decades, we have sought to develop methods to maintain, audit and improve the content of the MED, such that it remains true to its original design goals. This has resulted in a complex, multi-faceted process, with both manual and automated components. In this paper, we describe this process, with examples of its effectiveness. We believe that our process provides lessons for others who seek to maintain complex, concept-oriented controlled terminologies. © 2009 Elsevier Inc.