In this Original Report from the Journal of Clinical Oncology, the authors conducted a phase I trial of autologous CD19 CAR T cells in children and young adults (CAYAs) with relapsed or refractory B-cell acute lymphoblastic leukemia (B-ALL) to discover whether the role for allogeneic hematopoietic stem-cell transplant (alloHSCT) following CD19-CAR T-cell therapy can improve long-term outcomes.