BACKGROUND Post-transplant lymphoproliferative disorder (PTLD) is a potential complication of solid organ or bone marrow transplants. The main PTLD risk factors are: the Epstein-Barr virus (EBV), transplant type, and use of immunosuppressants. It mainly consists of an uncontrolled growth of lymphocytes in transplant recipients under chronic immunosuppressive therapy. About 85% of PTLDs are EBV-containing B-cell proliferations; 14% are T-cell proliferations, of which only 40% contain EBV; and the remaining 1% is NK-cell or plasmocyte proliferations. PTLD may present various clinical manifestations, from non-specific mononucleosis-like syndrome to graft or other organ damage resulting from pathologic lymphocyte infiltration. PTLD may manifest in the oral cavity. CASE REPORT The objective of this study was to present the case of a 13-year-old female living-donor liver transplant recipient, resulting from biliary cirrhosis caused by congenital biliary atresia, with exophytic fibrous lesions on buccal mucosa and tongue. Exophytic and hyperplastic lesion of oral mucosa were removed and histopathological examination revealed polymorphic PTLD. The patient underwent 6 cycles of CHOP chemotherapy and all the oral lesions regressed completely. CONCLUSIONS All oral pathological lesions in organ transplant recipients need to be surgically removed and histopathologically examined because they present an increased risk of neoplastic transformations such as PTLD.