Chimeric antigen receptor (CAR)-T cell therapies reprogram T cells to engage and eliminate cancer cells. Patients’ T cells are transduced in vitro using lentiviral or retroviral vectors containing a CAR transgene. Following infusion, CAR-T cells expand in vivo and may persist in the peripheral blood and bone marrow for years. Therefore, monitoring in vivo copies of the CAR transgene requires highly sensitive, validated analytical methods. Herein, we describe the validation of a qPCR assay to detect tisagenlecleucel transgene in patient samples. The limit of detection and lower limit of quantitation were 3.1 and 10 copies/200 ng genomic DNA, respectively, equivalent to ∼50 copies/μg genomic DNA and in alignment with US Food and Drug Administration guidance on bioanalytical method validation. The assay allowed quantitation of the tisagenlecleucel transgene over a wide dynamic range with a high degree of linearity, that is, 101–106 copies/200 ng genomic DNA (R2 ≥ 0.9988). Coefficients of variation of measured transgene copies ranged from 0.2% to 12.8%. A droplet digital PCR assay was performed as a method of validation and showed a strong correlation with the qPCR assay (R2 = 0.9980, p < 0.0001). This qPCR assay is being utilized to monitor tisagenlecleucel expansion and persistence in clinical trials. Display omitted Highly specific, sensitive, and quantitative analytical assays are needed to monitor the in vivo cellular kinetics of CAR-T cells in the patient after infusion. In this study, Davis and colleagues describe the validation results of a novel qPCR assay designed to specifically identify the tisagenlecleucel CAR transgene in patient samples.