Posttransplant lymphoproliferative disorders (PTLDs) of T-cell orgin are rare biologically heterogeneous diseases of mature lymphoid cells manifesting in immunosuppressed patients. Only a few cases of mycosis fungoides diagnosed post allogeneic hematopoietic cell transplant (alloHSCT) have been described so far. We present a patient with myelodysplastic syndrome (MDS) post matched unrelated donor alloHSCT who was on long-term immunosuppressive therapy due to graft versus host disease. Three years after an alloHSCT, she developed generalized erythroderma and peripheral blood lymphocytosis. Both skin biopsy and peripheral blood flow cytometry revealed atypical CD4+ T-cell population consistent with diagnosis of Sezary syndrome. Chimerism studies revealed 100% donor engraftment. Therapy with extracorporeal photopheresis resulted in complete response in blood and skin.