BACKGROUND Post-transplant lymphoproliferative disorder (PTLD) is a rare but highly fatal complication occurring after allogeneic hematopoietic cell transplantation (allo-HCT) or solid organ transplantation (SOT). Unlike SOT, PTLD after allo-HCT usually originates from the donor and is rarely accompanied by a loss of donor chimerism. CASE SUMMARY We report a case of Epstein-Barr virus positive PTLD manifesting as diffuse large B-cell lymphoma (DLBCL) with significantly decreased T-cell chimerism early after allo-HCT. A 30-year-old patient with acute myeloid leukemia underwent unrelated allo-HCT after first complete remission. Nearly 3 mo after transplantation, the patient developed cervical lymph node enlargement and gastric lesions, both of which were pathologically suggestive of DLBCL. Meanwhile, the patient experienced a significant and persistent decrease in T-cell chimerism. A partial remission was achieved after chemotherapy with single agent rituximab and subsequent R-CHOP combined chemotherapy. CONCLUSION The loss of T-cell chimerism and the concomitant T-cell insufficiency may be the cause of PTLD in this patient.