OBJECTIVE: To investigate early Epstein-Barr virus (EBV) reactivation and the outcome of preemptive therapy after allogeneic hematopoietic stem cell transplantation (allo-HSCT). METHODS: From January 2007 to January 2009, totally 277 patients after allo-HSCT were studied (haploidentical 116, unrelated 75, matched sibling 86). Conditioning regimens were mainly busulfan (BU) + cyclophosphamide (CY)/fludarabine (Flu) or total body irradiation (TBI) + CY/Flu. Antihuman thymocyte globulin (ATG) was added in haploidentical and unrelated transplants. Plasma EBV DNA was monitored once to twice weekly in the first 3 months after allo-HSCT with real time quantitative polymerase chain reaction (RQ-PCR). EBV viremia was diagnosed when EBV DNA was more than 5 × 10(2) copies/ml but without symptoms. Acyclovir (10 mg/kg, intravenous drip, 8 h) was used for preemptive therapy and immuno-suppressants were decreased if possible. RESULTS: Totally 33 patients (11.9%) developed EBV viremia with a median time at day 44 (day 19 to day 84). The incidences of EBV viremia in the transplants from matched sibling, haploidentical, unrelated donors were 0, 15.5%, 20.0%, respectively. There was no significant difference between haploidentical and unrelated transplants (P = 0.09), but much less EBV viremia was seen in matched sibling transplant (P = 0.001). Twenty of 33 patients (60.6%) had complete response to preemptive therapy. The median time to reach EBV DNA negative in plasma was 11 (4 - 56) d. The median duration of preemptive therapy was 21 (14 - 60) d. Both univariate and multivariate analysis indicated that haploidentical and unrelated transplants, acute graft versus host disease (GVHD) were the risk factors for EBV viremia. Two-year overall survival in the patients with EBV viremia was significantly lower than that without EBV viremia (54.2% vs 72.1%, P = 0.006). CONCLUSIONS: Our large clinical study has demonstrated that preemptive therapy with acyclovir that is guided by EBV viremia is effective in majority of the patients with high-risk for EBV reactivation after allo-HSCT, which may further decrease the risk for developing life-threatening EBV disease or post-transplantation lymphoproliferative disorder. Haploidentical and unrelated transplants, acute GVHD are the risk factors for EBV viremia which has negative impact on survival.