Background <p>CD19-directed chimeric antigen receptor (CAR) T-cell therapy has revolutionized cancer care for patients with relapsed or refractory (R/R) diffuse large B-cell lymphoma (DLBCL). Despite impressive responses seen with CAR T-cell treatment, nearly 30–50% of patients will relapse with very poor outcomes and the mechanism of relapse if oftentimes unknown. Next-generation sequencing (NGS) is a novel technology that can detect genomic biomarkers and provide insight into treatment resistance and cancer prognosis. This report highlights a case where NGS was able to detect the presence of the CAR T-cell construct in a patient after CAR T-cell therapy relapse.</p> Case presentation <p>We present a 20-year-old, White, male patient with R/R DLBCL who was treated with CD19 CAR T-cell therapy and relapsed approximately six months after infusion. Biopsy showed CD19-negative disease. Interestingly, NGS detected the presence of the CAR T-cell construct even after the patient progressed following CAR T-cell therapy, suggesting continued persistence of the CAR T-cells.</p> Conclusions <p>Prognosis of patients who relapse after CAR T-cell therapy for R/R DLBCL remains poor as mechanisms and predictors of relapse are not well understood. It is necessary to consider how novel technologies can be incorporated into the prognostication and monitoring of response to CAR T-cell therapy. In our case, the ability of NGS to distinguish the CAR T-cell product suggests that NGS may have the potential to ascertain CAR T-cell response and provide insight into the purported mechanism of relapse after CAR T-cell therapy. This report highlights a potentially new approach to monitoring patients with R/R DLBCL following CAR T-cell therapy using NGS technology.</p>