The health care sector still faces a large gap between the promoted and empirically demonstrated benefits of eHealth technologies. Medical departments in hospitals often build their own isolated software tools, resulting in silo applications with a low level of interoperability and in redundant, disorganized and inaccessible medical information. Based on a literature review, this paper derives design requirements for hospital's IS architectures. It argues that the Viable System Model, which has its origins in systems theory and cybernetics, provides a suitable basis to design an interoperable and scalable architecture for health information systems. Based on a design-oriented research approach, we develop a systems theory approach for IS planning in hospitals, which has been applied in a field study and evaluated in expert interviews. The evaluation confirms systems theory approach's appropriateness to cope with the specificities of Swiss hospitals and reveals additional avenues for refining the method.