Everywhere in the world, with aging, people accumulate more health problems. The more health problems they accumulate, the more difficult it is to recover to a healthier state, and the greater the risk of worsening and death (1). This increased risk is not the same for every older adult (2). People at greater risk compared with others of the same age are said to be frail (1–3). As populations age, the complexity of frailty challenges healthcare systems, especially intensive care provision (4–9). The complexity of frailty arises from both the number of problems that are active simultaneously and the tendency for intervention in one system (eg, diuretics to improve heart function) to adversely impact other systems (1,4,10). For instance, a patient with respiratory failure needs mechanical ventilation to improve oxygen supply but the high PEEP improving oxygenation can decrease return blood volume, thus inducing hypotension. Likewise, needed antibiotics can wipe out normal gut flora, predisposing to opportunistic infections. Many frail older adults can only live at home with the care provided them by family members, whose understanding and expectations can vary, making care planning difficult (11,12). These considerations commonly challenge contemporary management of critical illness (8,9,12–16). Briefly, intensive care units (ICUs) are challenged to understand which older adults are most likely to benefit (8,9,12). In these circumstances, considering frailty might help (1,3,4,9,13). Typically, ICUs employ several prognostic scores, focusing on acute episodes, consciousness, vital signs, and disease severity on admission. Most scores include age, but do not precisely assess comorbidity and prehospital functional status or disability, or heterogeneity of health status. Just as the degree of illness severity and organ compromise are measured by standard ICU prognostic scores, so too, in other settings, does the degree of frailty appear to influence the risk of adverse outcomes (1,4,17,18). In short, measuring not just the presence of frailty, but also its severity, might add value. A frailty index (FI) measures health deficit accumulation. It demonstrates that people are frail when they have more things wrong: on average, the more things that someone has wrong with them, the more likely they are to die (1,4,19,20). Briefly, the FI counts an individual’s health problems, broadly defined by biological and clinical characteristics, and expresses this as the ratio of the deficits present in the person to the total number of deficits considered in a given setting. The FI shows characteristic behavior: an age specific increase; strong association with mortality risk; and a quantifiable limit, beyond which score few survive (20–23). These characteristics have been verified in multiple studies (24–27). Of note, in institutionalized and clinical patients who are seriously ill, the FI score is more important than age in predicting survival (28–30). Even though the FI has been validated in acutely ill older adults admitted to hospital (1,28–31), and in prehospital care (32), less is known about its use in ICUs. Here, we hypothesized that ICU survival and overall mortality are closely related to the degree of frailty prior to ICU admission, so that people with higher FI scores are less likely to survive. The objectives were to: (a) construct an FI to evaluate the health status of older adults admitted to the ICU; and (b) examine the relationship between the FI score and survival to 30 and 300 days in comparison with several commonly employed ICU prognostic scores.