With the help of appropriate IT support, health care services can be executed in a more effective and secure way. In Sweden, the NPÖ (National Patients’ Översikt) stands for National Patients’ Overview. It is a platform where authorized health care providers can access comprehensive and continuous information about health care and patients’ situation, based on which care providers can offer safe and qualified services. The NPÖ project is focusing on the information sharing phase. In order to improve the efficiency and correctness of care services, the next step is that health care systems can offer clinical suggestions and warnings with the existing patients’ data and medication information. Clinical Decision Support Systems (CDSSs) are aimed to offer such assistance and are necessary to be integrated. But by now, there is no explicit architecture to guide Swedish government to implement the integration. Although some architectures have been proposed for integrating CDSSs in health information systems, those architectures are developed for certain use cases and cannot be adopted directly in NPÖ. An integration architecture which takes full consideration of NPÖ-adopting data types, message structures and interface types is needed. This thesis adopts constructive research method, which contains three main phases. First, related backgrounds about national electronic health care system, clinical decision supports system and integration techniques are introduced. Second, the integration architecture is constructed following service-oriented principles. Third, theoretical valuation work is finished by assessing system features and making interviews. This thesis takes advantage of service-oriented architecture to design an architecture with Clinical Decision Support (CDS) middleware for health care information system integration. With this structure, national electronic health care systems, such as NPÖ, can have interaction with various types of CDSSs to provide more efficient and secure health care. It offers united interfaces which enable different CDSSs with different developing platforms to communicate without obstacles. Unlike the existing CDSS integration architectures, the new one with CDS Middleware can provide maximized scalability. Evaluation work has been done from two aspects. Feature criteria and interviews with national health care system developers indicate that the architecture can contribute to the development of NPÖ, and future works such as involving security agents can be continued to optimize the results.