The importance of healthcare information technology has increased dramatically during the past few years. Researchers, developers, and companies have placed efforts to develop mobile, web, desktop, and enterprise e-health applications. This development raised the importance of interoperability and data exchange between e-health applications and Health Information Systems (HIS), which resulted in increased concerns for securing sensitive data and data exchange. This paper emphasizes standardizing a data exchange mechanism and interoperability between HISs and e-health applications across a cloud-based service. The proposed service provides a single point of entry for retrieving Electronic Medical Records (EMR) among the various HISs that a patient record is stored in and for any e-health application seeking such information. Moreover, this research proposes a unified secure platform that provides developers, health care providers, and organizations access to a framework to retrieve and manage medical records and Personal Health Records (PHR) among various subscribers. The proposed service and platform have been qualitatively evaluated among a sample of physicians, patients, and hospital administrators from the Faculty of Medicine, Alexandria University, Egypt. Results are promising and a prototype implementation is underway.