The clinical value of monitoring of Epstein-Barr virus (EBV) viraemia by quantitative polymerase chain reaction during 1 y was evaluated. 39 recipients of allogeneic hematopoietic stem cell transplantation (SCT) were followed. More than 100 EBV genome equivalents (gEq)/ml in blood or plasma were found in 16/39 patients (41%) at 34 d (range 1-139) post-transplant. Seven of these 16 patients developed EBV disease; 3 post-transplant lymphoproliferative disease (PTLD), 1 myelitis, 1 encephalitis and 2 reactivations with fever. EBV diseases were only found in the high-risk group among recipients of mismatched related or unrelated donor grafts or in patients who underwent reduced-intensity conditioning. In this group, 3/20 (15%) developed PTLD. Conditioning with antithymocyte globulin was significantly associated with EBV disease (p<0.01). EBV load in plasma was more strongly associated with EBV disease than viral load in blood. A cut-off level of 1000 gEq/ml plasma distinguished EBV disease from asymptomatic viraemia, but not PTLD from other EBV diseases. Weekly monitoring of EBV load in plasma in high-risk patients in the first 3 months following SCT seems to be of value for prediction of EBV disease. Therapy for PTLD including rituximab was evaluated during 2 y and showed response in 4/6 cases.