Davila and colleagues report a Phase I trial using CD19-specific chimeric antigen receptor (CAR)-engineered T cells for autologous transplant. The 16 patients with relapsed or refractory B cell acute lymphoblastic leukaemia (B-ALL) exhibited an 88% overall complete response rate. The authors also characterized diagnostic criteria and identified an already available test for the development of severe cytokine release syndrome, which is a complication of T cell transplant that requires therapeutic intervention.