To explore the burden of illness associated with infectious syndromes and to measure the associated use of antimicrobials in residential aged care facilities (RACFs). Design, setting and subjects: Retrospective analysis of data for January 2006 to December 2010 from an infection surveillance system covering residents of four co-located RACFs, with a total of 150 residential care beds, in Melbourne, Victoria. Main outcome measures: Number of episodes and incidence of health careassociated infection (HCAI); rate of antimicrobial use; prescribing concordance with McGeer criteria for infection; frequency of clinical specimen collection. Results: There were 1114 episodes of an infectious syndrome over 267 684 occupied bed-days (OBD), affording an average HCAI rate of 4.16 episodes/ 1000 OBD annually over 5 years (95 CI, 3.92-4.41). The mean rate of antimicrobial use was 7.07 courses/1000 OBD (range, 6.71-7.84). Around 40 of antimicrobial prescribing was for episodes that did not fulfil the McGeer criteria for clinical infection; this included about half of suspected urinary tract and upper respiratory tract infections (URTI), and about one-third of suspected lower respiratory tract and skin infections. Antimicrobials were routinely prescribed for URTI and bronchitis. Of all episodes treated with antimicrobials, 36 had documentation that a clinical specimen was obtained. Conclusions: The HCAI rate remained relatively stable over time. Routine surveillance and feedback of infection rates to the facilities did not result in a noticeable decrease of infection burden over time. It is of immediate concern that antimicrobials were being prescribed for a large proportion of suspected infections that did not meet criteria for clinical infection. Opportunities exist to further improve the use of antimicrobials in the RACF setting.