An essential element in defining the semantic of Web services is the domain knowledge. Medical informatics is one of the few domains to have considerable domain knowledge exposed through standards. These standards offer significant value in terms of expressing the semantic of Web services in the healthcare domain. In this paper, we describe the architecture of the Artemis project, which exploits ontologies based on the domain knowledge exposed by the healthcare information standards through standard bodies like HL7, CEN TC251 , ISO TC215 and GEHR. Artemis Web service architecture does not propose globally agreed ontologies; rather healthcare institutes reconcile their semantic differences through a mediator component. The mediator component uses ontologies based on prominent healthcare standards as references to facilitate semantic mediation among involved institutes. Mediators have a P2P communication architecture to provide scalability and to facilitate the discovery of other mediat...