The concept of frailty has been defined as a multidimensional syndrome characterized by the loss of physical and cognitive reserve that predisposes to the accumulation of deficits and increased vulnerability to adverse events. Frailty is strongly correlated with age, and overlaps with and extends aspects of a patient's disability status (that is, functional limitation) and/or burden of comorbid disease. The frail phenotype has more specifically been characterized by adverse changes to a patient's mobility, muscle mass, nutritional status, strength and endurance. We contend that, in selected circumstances, the critically ill patient may be analogous to the frail geriatric patient. The prevalence of frailty amongst critically ill patients is currently unknown; however, it is probably increasing, based on data showing that the utilization of intensive care unit (ICU) resources by older people is rising. Owing to the theoretical similarities in frailty between geriatric and critically ill patients, this concept may have clinical relevance and may be predictive of outcomes, along with showing important interaction with several factors including illness severity, comorbid disease, and the social and structural environment. We believe studies of frailty in critically ill patients are needed to evaluate how it correlates with outcomes such as survival and quality of life, and how it relates to resource utilization, such as length of mechanical ventilation, ICU stay and duration of hospitalization. We hypothesize that the objective measurement of frailty may provide additional support and reinforcement to clinicians confronted with end-of-life decisions on the appropriateness of ICU support and/or withholding of life-sustaining therapies.