The clinical computing environment at Columbia-Presbyterian Medical Center is organized around a centralized database of coded patient information collected from various ancillary sources. The Medical Entities Dictionary (MED) is the central repository for the controlled vocabulary used to encode the patient data. The MED is composed of terms used in the ancillary departments and, as such, changes in the source vocabularies must be maintained in the MED. The MED also contains some basic knowledge about the terms, and sophisticated maintenance tools have been developed that take advantage of this knowledge. This paper describes the success of the knowledge-based approach by describing the techniques used in two tasks: addition of a new vocabulary and maintenance of an existing one.