The advanced medical information systems usually consist of loosely-coupled interaction of independent systems, such as HIS, RIS, and PACS. To support easier information exchange between these systems and between hospitals, and to support new types of medical service such as teleradiology, it becomes essential to integrate separated medical information and allow them to be exchanged and retrieved through internet. This paper presents an integrated medical information system using XML. We analyzed HL7 and DICOM standard formats, and designed XML DTDs for HIS, RIS, PACS, and their integration. We extracted information from HL7 messages and DICOM files, and generated XML document instances and XSL stylesheets based on the proposed XML DTDs. We implemented the web interface for the integrated medical information system, which supports data sharing, information exchange and retrieval between two different standard formats. The proposed XML-based integrated medical information system will contribute to solve the problems of current medical information systems, by enabling integration of separated medical informations and by allowing data exchange and sharing through internet.