The objective of this study is to design and implement a common-gateway oriented mediator to solve the health data interoperability problems that exist among heterogeneous health information systems. The proposed mediator has three main components: (1) a Synonym Dictionary (SD) that stores a set of global metadata and terminologies to serve as the mapping intermediary, (2) a Semantic Mapping Engine (SME) that can be used to map metadata and instance semantics, and (3) a DB-to-XML module that translates source health data stored in a database into XML format and back. A routine admission notification data exchange scenario is used to test the efficiency and feasibility of the proposed mediator. The study results show that the proposed mediator can make health information exchange more efficient.