Relapse is a common cause of failure in patients with B-cell acute lymphoblastic leukaemia (B-ALL) after haploidentical haematopoietic stem cell transplantation (haplo-HSCT), and non-responders to donor lymphoblastic infusion after HSCT have a very poor prognosis. Although donor-derived CD19-directed chimeric antigen receptor-modified (CAR) T cells can potentially cure leukaemia, their effectiveness and safety have not been confirmed in relapsed B-ALL cases after haplo-HSCT. Between January 2015 and January 2017, two and four patients each received one and two infusions of CAR T cells from haplo-HSCT donors. Five (83·33%) achieved minimal residual disease (MRD)-negative remission; one patient was discharged automatically without evaluation after developing severe thrombotic microangiopathies. Four of five responsive patients relapsed after 2-7 months, and one died of sepsis following MRD-negative remission after a second infusion. None of the other second infusion recipients achieved a second complete remission. Five patients (83·33%) experienced eight courses of grade 1-3 cytokine release syndrome; two were treated with tocilizumab. Two (33·3%) and one patient developed grade 2 and 3 acute graft-versus-host disease (aGVHD), respectively; the former was controlled with glucocorticoids. Donor-derived CAR T-cell infusion seems be effective and safe for relapsed B-ALL after haplo-HSCT, although larger clinical studies are needed.