To facilitate the provision of clinical decision support (CDS), the Unified Medical Language System (UMLS) was leveraged to implement a terminology Web service. Supported functions include inter-vocabulary translation and the identification of concepts subsumed by a parent concept. Currently, the service is being used to aid the specification of clinical concepts within CDS knowledge modules. Insights gained from this process are discussed, including the limitations of using the UMLS to fulfill CDS terminology needs.