The present paper describes six cases of lymphoproliferative disorders (LPD) occurring after bone marrow transplantation. Treatments were ineffective and disease was rapidly fatal in all patients, although immunotyping of cells in blood, bone marrow or cerebrospinal fluid was helpful to establish the diagnosis of LPD. Monoclonality was demonstrated in the 4 cases which it was possible to analyse. Herpes virus genome was present in tumoral cells of 4 in 4 cases tested for EBV, one in 3 cases tested for CMV, one in 3 cases tested for HHV6 and 3 in 3 cases tested for HSV. Patients developing LPD should benefit from earlier diagnosis and new therapeutic approaches such as donor lymphocyte infusions, while further studies are necessary to elucidate the role of Herpes viruses in the pathogenesis of LPD.