Background: EBV⁺ PTLD is often a fatal disease. Tabelecleucel is an investigational off-the-shelf, allogeneic T-cell immunotherapy specific for EBV antigens. Functional in vivo expansion approximated by a limiting dilution analysis of EBV-CTLp has previously been correlated to clinical response. Methods: We conducted analyses of reported peak circulating EBV-CTLp levels following tabelecleucel administration in correlation to response and safety across two studies (N=42) in rituximab refractory or relapsing EBV⁺ PTLD following HCT or SOT. Results: Two cohorts were defined by the lowest EBV-CTLp quartile (n=11) and the combined upper 3 quartiles (n=31). Twenty-five of 31 subjects (80.6%) in upper quartiles exhibited a clinical response compared to 3 of 11 subjects (27.3%) in the lowest quartile. Eighteen of 20 complete responses and 7 of 8 partial responses demonstrated peak CTLp values in the upper quartiles of the population. Overall survival (OS) rate in subjects exhibiting CTLp values in the upper quartiles was 83.9% (95% CI: 65.5-92.9%) at 1 year and 66.1% (95% CI: 45.9-80.2%) at 2 years, compared to 18.2% (95% CI: 2.9-44.2%) in the lowest quartile over the same periods. OS is commensurate with a hazard ratio of 0.168 in favor of subjects with higher EBV-CTLp (95% CI: 0.067-0.425; Log-Rank Test p-value <0.001). Subjects demonstrating clinical responses experienced median CTLp values of 36.2 CTLp/ml (15.6-155 interquartile range), whereas non-responders exhibited significantly lower (p=0.001, based on Wilcoxon Rank Sum test) median CTLp of 0.07/ml (0.01-5.5 interquartile range). Tabelecleucel was well tolerated across both studies, with a safety profile in each group consistent with the underlying clinical condition of patients. Conclusions: These analyses illustrate the correlation of peak EBV-CTLp to clinical benefit in subjects receiving tabelecleucel. Significant correlation of EBV-CTLp with ORR and OS are consistent with potential use as a functional biomarker for the expansion of EBV-specific T-cells in the treatment of EBV⁺ PTLD in the rituximab refractory/relapsing setting. Clinical trial information: NCT01498484