Background Minimal residual disease (MRD)-directedmodified donor lymphocyte infusion (mDLI) is used to treat relapse after hematopoietic stem cell transplantation (HSCT).For patients who experience an unsatisfactory response tomDLI,relapse is usually inevitable.Therefore,we sought to evaluate the efficacy ofinterferon α therapy in these patients.Methods Regular MRD monitoring was carried out after the HSCT.The patients who were MRD-positive underwent mDLI.Patients with an unsatisfactory response to mDLI received interferon α therapy (3 million units,twice weekly) with regular monitoring of MRD.To ensure the immunomodulatory effects of interferon α,immunosuppressant treatment would be stopped before interferon α treatment.Results Five patients with an unsatisfactory response to mDLI treatment received interferon α (3 had t(8;21) chromosomal translocation acute myeloid leukemia,and 2 had common acute leukemia).They had significantly reduced or resolved MRD.Four patients developed chronic graft-versus-host disease.Two of the 5 patients reported transient fevers,and no significant bone marrow suppression was observed.All of them were in continuous complete remission after interferon α treatment.The median survival time was 469 days (range 368-948 days).Conclusions In patients with an unsatisfactory response to MRD-directed mDLI,interferon α may directly or indirectly induce the graft-versus-leukemia effect to improve mDLI efficacy and clear MRD.