We are developing an in-house Picture Archiving and Communication System (PACS) targeted to the centralized storage of images acquired from our computer-assisted imaging modalities and to the display of these images on multi-modality viewing stations. The central image database is distributed between two system modules with image processing capabilities located at the viewing stations. The first module of the image database is an image filing subsystem used for the storage and retrieval of complete image files. In our initial work, this module consists of a Data General MV/6000 computer system with 1.16 Gbytes of on-line disk storage. However, more efficient dedicated filing systems may be substituted for this general purpose computer in future revisions. The second module is an image file indexing subsystem which has been implemented on a DEC PDP-11/44 computer and is tightly integrated with our MARS II (ADAC Laboratories) Radiology Information System. These two image database modules communicate via low-speed, serial communications lines. This report focuses on our developmental work on the image file indexing subsystem and its communications protocol with the image filing subsystem. The image file indexing subsystem automatically inserts image file locators for studies referenced by patient when an image is transmitted from the acquisition device (eg, CT) to the image filing subsystem. It also locates image files at the request of a viewing station user based on patient name, physician name, or study type for either read or unread studies. Other capabilities include cross-referencing patient attributes with the MARS II system, deleting studies based on predefined criteria, storing requests for hard copy, storing user selected image processing attributes for individual images, flagging the archival status of an image, and complete managerial functions. This central-ized image filing and indexing system comprises what will become the central element of our PACS.