EBV viral load (EBV-VL) in PBMC was prospectively determined by semi-quantitative PCR in 85 stem cell transplants (40 genoidentical, 45 non-genoidentical) in order to characterize the kinetics of EBV-VL and to assess the ability of this measure to predict the development of EBV-induced lymphoproliferative disease (EBV-LPD). PCR was performed prior to and after transplantation. An EBV-VL >300 copies/microg DNA was chosen as the threshold for risk of developing an EBV-LPD. Two hundred and fifty-eight EBV-VL measures were evaluable. Five patients (5.9%) developed an EBV-LPD. All had an elevated EBV DNA peak level before EBV-LPD. Fifteen out of 80 recipients (18.7%) without EBV-LPD had EBV levels over 300 copies/microg DNA at least once during the follow-up. Overall, the manifestation of at least one EBV-VL over 300 copies/microg DNA during the entire follow-up demonstrated a sensitivity, specificity, positive and negative predictive value for the diagnosis of EBV-LPD of 100%, 81%, 25% and 100%, respectively. In patients without EBV-LPD, HLA incompatibility, grade > or = II acute GVHD and use of an unmanipulated graft were significantly associated with an EBV-VL >300 copies/microg DNA. This strategy appears sensitive for the diagnosis of EBV-LPD but its positive predictive value has to be improved in order to guide pre-emptive therapy.