Abundant evidence suggests a relationship between antimicrobial resistance and use, including models, consistent associations between resistance and antimicrobial use in hospitals, concomitant variation in resistance as antimicrobial use varies, and a dose-response relationship for many pathogen/antimicrobial combinations. Much of the evidence is from studies performed in single hospitals. Most multicenter studies on resistance have not included data on antimicrobial usage. Despite this substantial body of evidence, some studies have not demonstrated an association between antimicrobial resistance and use, suggesting other contributing factors such as cross transmission, interhospital transfer of resistance, a community contribution to resistance, or a complex relationship between resistance and the use of a variety of antimicrobials. Understanding the problem of antimicrobial resistance in a hospital cannot be achieved without knowledge of the hospital's pattern of antimicrobial use.