The performance of a plasma real-time PCR (cytomegalovirus [CMV] PCR kit; Abbott Diagnostics) was compared with that of the antigenemia assay for the surveillance of active CMV infection in 42 allogeneic hematopoietic stem cell transplantation (Allo-SCT) recipients. A total of 1,156 samples were analyzed by the two assays. Concordance between the two assays was 82.2%. Plasma DNA levels correlated with the number of pp65-positive cells, particularly prior to the initiation of preemptive therapy. Fifty-seven episodes of active CMV infection were detected in 37 patients: 18 were defined solely by the PCR assay and four were defined on the basis of the antigenemia assay. Either a cutoff of 288 CMV DNA copies/ml or a 2.42-log 10 increase of DNAemia levels between two consecutive PCR positive samples was an optimal value to discriminate between patients requiring preemptive therapy and those not requiring therapy on the basis of the antigenemia results. The real-time PCR assay allowed an earlier diagnosis of active CMV infection and was a more reliable marker of successful clearance of CMV from the blood. Analysis of the kinetics of DNAemia levels at a median of 7 days posttreatment allowed the prediction of the response to CMV therapy. Two patients developed CMV colitis. The PCR assay tested positive both before the onset of symptoms and during the disease period. The plasma real-time PCR from Abbott is more suitable than the antigenemia assay for monitoring active CMV infection in Allo-SCT recipients and may be used for guiding preemptive therapy in this clinical setting.Preemptive antiviral therapy based on the detection of cytomegalovirus (CMV) in blood has been shown to significantly reduce the incidence of CMV disease in the early posttransplant period following allogeneic hematopoietic stem cell transplantation (Allo-SCT) (2, 7). The pp65 antigenemia assay has been adopted by many transplant centers as the "gold standard" for monitoring active CMV infection and guiding preemptive therapy in this clinical setting. The antigenemia assay, however, requires rapid processing of specimens, is labor-intensive, cannot be automated, is subjective in reading, and is unfeasible during periods of severe neutropenia. In addition, it does not reflect the viral load in the blood compartment accurately (12) and sometimes displays negative results in the presence of CMV disease (2,31,32,36).Quantification of CMV DNA in blood by PCR is emerging as an alternative to the antigenemia assay and may soon become the standard for the surveillance of CMV infection in Allo-SCT recipients. Viral load quantification in blood allows early detection of active CMV infection, close monitoring of the response to antivirals, prediction of the risk of viremic relapse, emergence of resistant strains, and eventual development of CMV disease (8,31). In addition, it may be safely used for guiding preemptive therapy (11,16,23,24,32,37). Nevertheless, there is some controversy as to which is the optimal clinical specimen (plasma, whole blood, or leukoc...