The SNUH started a project to innovate the hospital information facilities. This project includes installation of high sped hospital network, development of new HIS, OCS, RIS and PACS. This project aims at the implementation of the first total hospital information system by seamlessly integrating these systems together. To achieve this goal, we took three-tiered systems integration approach: network level, database level, and workstation level integration. There are 3 loops of networks in SNUH: proprietary star network for host computer based HIS, Ethernet based hospital LAN for OCS and RIS, and ATM based network for PACS. They are linked together at the backbone level to allow high speed communication between these systems. We have developed special communication modules for each system that allow data interchange between different databases and computer platforms. We have also developed an integrated workstation in which both the OCS and PACS application programs run on a single computer in an integrated manner allowing the clinical users to access and display radiological images as well as textural clinical information within a single user environment. A study is in progress toward a total hospital information system in SNUH by seamlessly integrating the main hospital information resources such as HIS, OCS, PACS. With the three-tiered systems integration approach, we could successfully integrate the systems from the network level to the user application level.