Background: Frailty is associated with a higher risk for adverse outcomes after aortic valve replacement (AVR) for severe aortic valve stenosis, but whether or not frail patients have differential benefit from transcatheter (TAVR) vs. surgical (SAVR) AVR is uncertain. Methods: We linked adults ≥ 65 years old in the US CoreValve High Risk (HiR) or Surgical or Transcatheter Aortic-Valve Replacement in Intermediate Risk Patients (SURTAVI) trial to 100% Medicare claims 2/2/2011-9/30/2015. Two frailty measures, a deficit-based (DFI) and phenotype-based (PFI) frailty index, based on the Rockwood and Fried definitions of frailty respectively, were generated for linked individuals. The absolute and relative treatment effect of TAVR vs. SAVR were evaluated within frailty index (FI) tertiles for the primary endpoint of death and non-death secondary outcomes, as defined by the trials, using multivariable Cox regression. Functional status decline was additionally in TAVR vs. SAVR by FI tertile using multivariable logistic regression. Results: Of 1,442 (linkage rate = 60.0%) individuals in the linked HiR and SURTAVI trials, 741 (51.4%) individuals received TAVR and 701 (48.6%) received SAVR (mean age 81.8 ± 6.1 years, 44.0% female, mean Society of Thoracic Surgeons risk score 5.9 ± 2.7). Those in higher FI tertiles had a greater burden of comorbidities. Though the rates of death in the highest FI tertiles (DFI 38.6%, PFI 33.8%) were 2-3-fold higher than the lowest tertile (DFI 14.8%, HR 2.86, 95% CI 2.15-3.79, p < 0.001; PFI 17.9%; HR 2.05, 95% CI 1.58-2.67, p < 0.001), there were no significant differences in the relative or absolute treatment effect of SAVR vs. TAVR across FI tertiles for all death, non-death, and functional outcomes (all interaction p-values > 0.05). Results were overall consistent across individual trials and frailty definitions ( Figure ). Conclusions: Among CoreValve trial participants linked to Medicare claims, two different frailty indices based on Fried and Rockwood definitions identified individuals at higher risk of death and functional decline but no differential benefit from TAVR vs. SAVR.