Objective: To evaluate the impact of frailty, measured using the Canadian Study of Health and Aging Clinical Frailty Scale, on outcomes of older people hospitalized with acute illness. Method: Consecutive patients were randomly allocated to a model development sample or a model validation sample. Multivariate analyses were used to model in-hospital mortality, new nursing home placement, and length of stay. Variables selected in the development samples were tested in the validation samples. Results: The mean age of all 2,125 patients was 82.9 years. Most (93.6%) were admitted through the emergency department. Frailty predicted in-hospital mortality (odds ratio [OR] = 2.97 [2.11, 4.17]), new nursing home placement (OR = 1.60 [1.14, 2.24]), and length of hospital stay (hazard ratio = 0.87 [0.81, 0.93]). Discussion: Frailty is a strong predictor of adverse outcomes in older people hospitalized with acute illness. An increased awareness of its impact may alert clinicians to screen for frailty.