Background: Frailty has been identified as a marker of physiologic reserve, and a more accurate predictor of adverse postoperative outcomes compared with age. Although many definitions of frailty exist, recently a clinical predictive rule, the “modified frailty index”(mFI), has been developed utilizing administrative data to predict adverse outcomes in the lung cancer population undergoing lobectomy. Our goal was to validate this clinical rule among patients with bladder cancer undergoing cystectomy. Methods: Patients undergoing cystectomy were identified from the National Surgical Quality Improvement Program (NSQIP) participant use files (2006-2011). The mFI was defined as in prior studies with 11 variables based on mapping the Canadian Study of Health and Aging Frailty Index to NSQIP comorbidities and activities of daily living (ADL)s. These 11 variables each received 1 point, and the sum was divided by 11 for a fraction between 0 and 1. Univariate χ², independent sample t-test, and logistic regression analyses were performed where appropriate. Results: Of the 1,302 cystectomy patients identified, 30% had mFI of 0, 40% had mFI of 0.09, 21% had mFI of 0.18, and 9% had mFI ≥0.27. Overall, 56% of patients experienced a Clavien complication. Patients with mFI ≥0.27 were older ( 72 vs 64 yrs)and more likely to be smokers (54%) compared with mFI of 0 (30%, p<0.01). Mean operative times (342-349 minutes) were similar across mFI indices. Reoperation (5% vs 8.5%) and readmission (20.5% vs 25%) were higher when mFI =0 compared with mFI≥0.27 (P<0.01). Clavien 4 and above complications occurred in 9.1% (36/396), 10.1% (53/526), 12.9 % (35/270) and 13.6% (15/110) among patients with an mFI of 0, 0.09, 0.18, and ≥0.27, respectively (p=0.05). Similarly, the overall mortality rate increased from 2.5% in the lowest frailty index group to 5.4% in the highest. Conclusions: Among patients undergoing cystectomy, the modified frailty index can identify those patients at greater risk for severe complications, readmissions, and mortality. Given that bladder cancer is increasing in prevalence particularly among the elderly, pre-operative risk stratification is crucial to inform decision-making.