[Show abstract][Hide abstract] ABSTRACT: Inappropriate antimicrobial treatment (defined as use of antimicrobial agent to which a pathogen is resistant) or a delay in starting appropriate treatment are both associated with increased morbidity and mortality. Studies of ventilator-associated pneumonia, intra-abdominal infections or bacteraemia document higher mortality in patients who received inappropriate therapy. In addition, the outcome in patients switched from inappropriate to appropriate therapy is better than for patients who remained on inappropriate therapy, but the benefit is not as great as for those who were started on appropriate therapy initially. While inappropriate therapy undoubtedly has an important influence on outcomes, it needs to be considered in the context of other patient risk-factors, such as co-morbid conditions, severity score measures, and functional status. When assessing the impact of inappropriate therapy on outcomes such as length of hospital stay, it is important to be as precise as possible about the time of onset of infection. Failure to do so may lead to inaccurate estimation of the effect of inappropriate therapy. While the likelihood that resistant pathogens can increase costs throughout the healthcare system is generally recognised, an under-appreciated aspect of resistance is its consequences for patients and their carers. Initiatives are underway to gauge the impact of resistance and strategies to combat its spread.