Healthcare professionals need access to accurate and complete healthcare records for effective assessment, diagnosis and treatment of patients. The non-interoperability of healthcare information systems means that inter-enterprise access to a patient's history over many distributed encounters is difficult to achieve. The ARTEMIS project has developed a secure semantic web service infrastructure for the interoperability of healthcare information systems. Healthcare professionals share services and medical information using a web service annotation and mediation environment based on functional and clinical semantics derived from healthcare standards. Healthcare professionals discover medical information about individuals using a patient identification protocol based on pseudonymous information. The management of care pathways and access to medical information is based on a well-defined business process allowing healthcare providers to negotiate collaboration and data access agreements within the context of strict legislative frameworks.