Extramedullary plasmocytoma (EMP) is a rare cell neoplasm most frequently localised in the upper respiratory tract. We report the case of a 43 year-old-man, with an unusual presentation of EMP developing in the mediastinum, two years after a diagnosis of solitary plasmocytoma of the bone which was successfully treated by local irradiation. In this aggressive presentation, we decided to perform an autologous hematopoietic stem cell transplantation. Two months after transplantation, CT scan showed disappearance of the mediastinal mass and immunofixation of the serum was normal. Selected cases of diffuse EMP, could benefit from intensive treatment followed by autologous hematopoietic stem cell transplantation.