Lacking matched sibling donors, a larger portion of patients (pt) who require allogeneic bone marrow transplant (BMT) are receiving grafts from alternative donors increasing the risk of Epstein-Barr virus associated 6 Cell lymphoproBferative disorders (EB-LPDj Other know risk factors include the use of T-cell depletion (TCD), post-BMT immunotherapy (IT) using cyclosporin and antilymphocyte globulin (ATG) and graft-versus host dteease (GVHD). Thus pts receiving partially mismatched related donor (PMRD) BMT in our Center could be at high risk for EB-LPD. and often rapidly fatal complication. We performed a retrospective evaluation of 163 consecutive pts who underwent BMT between 2/93 and 1/96 using grafts which were 1 to 3 major-HLA antigen mismatched. Marrow grafts were T-depieted in vitro with complement mediated, CD-3 targeted monoclonal antibodies, using either T10B9 (IgM) [n=97] or OK-3 (IgG) [n=66J. In vivo T cell IT included ATG pre-BMT (n=86) and post-BMT (n=183), high-dose Methylprednisolone and cyclosporin. The incidence of grade II-IV acute GVHD was <20%. Six (3 male, 3 female) pts (3.3%) developed EB-LPD 48-253 (median 107) days after BMT. There were equal cases observed in T10B9 vs OKT-3, and in ATG post-BMT vs both pre- and post-BMT groups (3/97 (31%] vs 3/66 (3.5%), both respectively; NS). Clinically only 1 affected pt had developed >grade I acute GVHD and received a 2nd course of ATG. All pts received alpha-inlerferon, Acyclovir, intravenous immunoglobulin and reduction in immunosuppressive therapy. One pt underwent resection and 1 received alpha-inlerferon, Acyclovir, intravenous immunoglobulin and reduction in immunosuppressive therapy. One pt underwent resection and 1 received donor leukocyte infusion. Three patients died 13, 21 and 44 days after presentation and the other 3 are alive with follow-up between 20-750 days. Our study demonstrated an unexpected low Nicidence of EB-LPD which may be explained by the low incidence of acute GVHD suggesting that prevention of GVHD may be more important than the intensity of IT (TCD ATG+CYS) in the successful recovery of immunological regulation following PMRD BMT.