The aim of this study was to determine whether the association of small label size of the surgical valve with increased mortality after transcatheter valve-in-valve (ViV) implantation is, at least in part, related to pre-existing prosthesis-patient mismatch (PPM) (i.e., a bioprosthesis that is too small in relation to body size). Transcatheter ViV implantation is an alternative for the treatment of patients with degenerated bioprostheses. Small label size of the surgical valve has been associated with increased mortality after ViV implantation. Data from 1,168 patients included in the VIVID (Valve-in-Valve International Data) registry were analyzed. Pre-existing PPM of the surgical valve was determined using a reference value of effective orifice area for each given model and size of implanted prosthetic valve indexed for body surface area. Severe PPM was defined according to the criteria proposed by the Valve Academic Research Consortium 2: indexed effective orifice area <0.65 cm2/m2 if body mass index is <30 kg/m2 and <0.6 cm2/m2 if BMI is ≥30 kg/m2. The primary study endpoint was 1-year mortality. Among the 1,168 patients included in the registry, 89 (7.6%) had pre-existing severe PPM. Patients with severe PPM had higher 30-day (10.3%, p = 0.01) and 1-year (unadjusted: 28.6%, p < 0.001; adjusted: 19.3%, p = 0.03) mortality rates compared with patients with no severe PPM (4.3%, 11.9%, and 10.9%, respectively). After adjusting for surgical valve label size, Society of Thoracic Surgeons score, renal failure, diabetes, and stentless surgical valves, presence of pre-existing severe PPM was associated with increased risk for 1-year mortality (odds ratio: 1.88; 95% confidence interval: 1.07 to 3.28; p = 0.03). Patients with severe PPM also more frequently harbored high post-procedural gradients (mean gradient ≥20 mm Hg). Pre-existing PPM of the failed surgical valve is strongly and independently associated with increased risk for mortality following ViV implantation. Display omitted