The objective of our study was to support algorithmic recommendations for the appropriate sizing of picture archiving and communication system (PACS) caches. For a large PACS installation, a detailed analysis of current radiographic (CR), CT and MRI studies (n=400 consecutive studies, respectively) was performed. In this process every available prior examination was also considered. A new algorithmic procedure for appropriate sizing of PACS caches was developed and applied to the given cache implementation. The number of all priors was 7.6±12.3. Of them, 61% were relevant priors with an average age of 203±385 days. A basic cache (BC) that covers 12 months of current imaging results in a pre-fetching quote of 15.4% (8.6% for 24 months). In the PACS installation examined, a minimal threefold increase in cache capacity was recommended. Studies that are retrieved, prior to viewing, from the long-term archive require additional space in the extended cache (EC). An intimate and mutual interaction between hospital information system (HIS), radiology information system (RIS) and PACS minimizes this requirement and increases the time during which actual image material from the BC is available online. A basic cache size covering actual imaging of 12 months up to 24 months is recommended. The parameters governing the individual dimensions of both cache spaces and a mathematical algorithm are demonstrated.