Cytomegalovirus (CMV) disease is a frequent cause of morbidity and mortality following allogeneic hematopoietic stem cell transplantation. In order to investigate the relationships between antigenemia, high-dose acyclovir (HDACV) prophylaxis, preemptive ganciclovir (GCV) therapy, and outcomes, we analyzed the records of 105 patients, including both pediatric and adult populations, who underwent allogeneic peripheral blood stem cell transplantation (PBSCT) and who were at risk for CMV reactivation and disease (both recipient and donor seropositive). All received HDACV until neutrophil engraftment, but prophylaxis was continued till post-transplant day 180 only in pediatric patients in conjunction with weekly CMV pp65 antigenemia monitoring. Antigenemia-guided preemptive strategy with GCV was used for all patients. CMV antigenemia developed in 45 patients (42.9%) and CMV disease in 13 (12.4%). The frequencies for antigenemia were 31.3 and 63.2% in pediatric and adult groups (P = 0.002). All CMV diseases were in the adult group (P<0.001). Age at transplantation, underlying disease, long-term HDACV prophylaxis and acute graft versus host disease (aGVHD) were all found to be a significant risk factors for antigenemia. All of these factors other than aGVHD and conditioning regimen were also the significant risk factors for CMV disease. However, when we analyzed the pediatric and adult patients separately, dropping "long-term HDACV prophylaxis," none of these parameters were significant risk factors for CMV disease. In conclusion, we hypothesize that long-term HDACV prophylaxis in the GCV era results in a low incidence of CMV reactivation and disease in patients undergoing PBSCT.