Preemptive therapy in hematopoietic cell transplantation is initiated by a diagnostic technique at first detection of cytomegalovirus (CMV). The aim of this study was to use the viral dynamics of CMV DNA viral to start preemptive therapy, and to prospectively compare the CMV viral load kinetics to pp65 antigenemia. Two hundred sixty-three blood samples were collected prospectively from 93 patients. All clinical decisions regarding use of preemptive therapy were based on CMV antigenemia. Based on the positivity of the antigen assay and clinical CMV outcome in allotransplant patients, an optimal threshold of 3.05 log 10 (1,130 copies/ml) was found to discriminate patients who required preemptive therapy and those who did not (sensitivity, 71%; specificity, 65%). A DNAemia level increase of 2.24 log 10 (174 copies/ml) per day was the optimal threshold to discriminate between patients who required preemptive therapy and those who did not (sensitivity, 93%; specificity, 43%). Sensitivity of PCR assay was 92.4% compared with 39% for the antigen assay (P < 0.001). A standardized real-time PCR assay is more appropriate than the antigen assay for detecting CMV. It allowed earlier diagnosis of active CMV infection and monitoring of the response to anti-CMV treatment. J. Med. Virol. 83:490–495, 2011. © 2011 Wiley-Liss, Inc.