From the user's point of view -- and that should be the starting point for the development of any information system -- a full-scale PACS has to do at least two things: (a) offer access to all available patient data, both the pictorial and the alphanumeric data, from the workstation, and (b) the response time related to the retrieval and display of these data must at least be acceptable. In order to be able to meet both requirements, the PACS must be linked with the HIS/RIS. For the first requirement, the HIS/RIS will in general be the main source for supplying alphanumeric patient data. The second demand can only be fulfilled if the PACS is able to manage its image database properly. A strategy which provides a way to optimize PACS image management using information generated elsewhere (e.g., by the HIS/RIS) is called prefetching. In this paper, a model of prefetching is presented, and a number of possible parameters for the prefetching algorithm are discussed. Finally, a method for evaluating the effectiveness of the algorithm is proposed and illustrated with the first results of a pilot study carried out in the Leiden University Hospital.