Tisagenlecleucel is a CD19-directed autologous chimeric antigen receptor (CAR) T-cell therapy. Quantitative polymerase chain reaction assays are highly sensitive in defining in vivo kinetics by measuring CAR transgene in peripheral blood. This study aimed to identify clinically meaningful CAR T-cell blood levels that correlated with response/relapse. In pediatric/young adult patients with relapsed/refractory (R/R) B-cell acute lymphoblastic leukemia (B-ALL), maximum CAR T-cell blood levels were higher in patients with ongoing complete remission and in patients with CD19- relapse relative to those with CD19+ relapse. In adult patients with R/R diffuse large B-cell lymphoma (DLBCL), no apparent association between in vivo kinetics and response was noted, with a wide range of transgene levels at relapse. In B-ALL, patients with B-cell aplasia sustained >6 months had higher CAR T-cell expansion relative to those with early B-cell recovery (BCR) (<6 months after infusion); however, a definitive cutoff for BCR-associated expansion level could not be identified. In most patients with B-ALL, BCR >6 months maintained favorable responses. However, early BCR could not be confirmed as a potential indicator of relapse due to high censoring from transplant, following presumed risk of relapse. However, allografting in these patients may potentially mitigate the poor prognosis related to early BCR. In DLBCL, BCR was not associated with relapse. These findings suggest that blood CAR transgene levels may be associated with long-term responses; however, they lack robust predictive potential for relapses. As reported earlier, next-generation sequencing for minimal residual disease appears to be a reliable biomarker predictive of relapse for B-ALL.