Second cancers and particularly postransplant lymphoproliferative disorders (PTLDs) are extremely rare in patients undergoing autologous peripheral blood stem cell transplantation (auto-SCT). We report the case of clonally rearranged T-cell expansion which occurred after auto-SCT for Multiple Myeloma (MM). Does asymptomatic clonal T-cell large granular lymphocytic proliferation, in our experience, represent either a secondary cancer after auto-SCT or clonal T cell aberration or derive from expansion of coexisting undetected small-sized clone of T cells?