Cytomegalovirus (CMV) remains a major cause of infection in recipients of hematopoietic stem cell transplants (HSCT) and results in significant mortality and morbidity. We present the results of CMV pp65 antigenemia-guided, risk-adapted preemptive therapy aimed at preventing CMV disease in allogeneic HSCT. Preemptive ganciclovir treatment was started when more than 5 CMV antigen-positive cells were detected in the low-risk group (with grade 0-I acute GVHD and matched related HSCT) and when any antigen-positive cells were seen in the high-risk group (with grade II-IV acute GVHD or matched unrelated HSCT). At least 1 episode of antigenemia was observed in 53 (59.6%) of 89 patients before day 100, and preemptive therapy was performed in 33 patients. CMV disease occurred in 6 patients (5 in the high-risk group and 1 in the low-risk group), and late CMV disease developed in 4 patients. Only 1 patient died of CMV pneumonitis before day 100. Neutropenia was observed in 51.5% of ganciclovir-treated patients, and coinfection/superinfection was observed in 42.4%. A strategy of ganciclovir treatment focusing on patients at higher risk could reduce the toxicity from the antiviral drug and be cost-effective. Extended surveillance for CMV disease using more sensitive diagnostic methods is necessary in high-risk patients.