The use of Electronic Health Records (EHR) is wide spread in healthcare. One of the most challenging tasks for EHR systems is to achieve computable semantic interoperability. To address EHR interoperability, a number of standardization efforts are progressing, however these standards are either incomplete in terms of functionality or lacking specification of precise meaning of underlying data. This paper describes an interoperable EHR framework that uses an ontology-based approach to facilitate exchange of information and knowledge among EHR. Based on the proposed framework, an interoperability scenario between a Personal Health Record System, an EHR and a Laboratory System is described.