Cytomegalovirus infection (CMV-I) and CMV related diseases (CMV-D) occurred after allogeneic hematopoietic stem cell transplantation (Allo-HSCT) seem to be with high morbidity and mortality. This study is a retrospective analysis of the incidence of CMV infection and diseases in Allo-HSCT patients known to be CMV seropositive before transplantation. To review the efficacy of CMV pp65 antigen-guided ganciclovir prophylaxis in preventing CMV infection and to search the optimal determination methods, 45 consecutive Allo-HSCT patients have been observed. Using the CMV pp65 antigenemia assay and serological analysis monitored blood samples from 23 patients with chronic myeloid leukemia (CML), 7 acute myeloblastic leukemia (AML), 6 acute lymphoblastic leukemia (ALL); other: 4 myelodysplastic syndrome (MDS), 3 non-Hodgkin's lymphoma (NHL) and 2 aplastic anemia. Forty-three patients received HLA-identical siblings transplantation and 2 from their HLA-haploidentical donors. Forty-five cases included Allo-PBPCT (38 cases), Allo-BMT (2 cases) and Allo-PBPCT + BMT 5 cases. Before transplantation, all donors/recipients have taken CMV serological detection. All donor/recipients were CMV IgG positive and one donor and one recipient with CMV IgM positive, respectively. After transplantation, all patients developed CMV antigenemia during monitoring period. Twenty-five patients developed CMV related interstitial pneumonia (CMV-IP). Patients have been followed from 6 to 28 months (median of 18 months) after transplantation. The patients who received preemptive therapy had a significantly better outcome than patients who did not received preemptive therapy. CMV related mortality was 1/29 cases in preemptive group vs. 12/16 cases in non-preemptive group. The results suggest that prompt and early institution of effective therapy with ganciclovir upon detection of CMV pp65 antigenemia, provides optimal protection against progress of CMV disease for patients undergoing Allo-HSCT.