Cytomegalovirus (CMV) disease remains an important cause of morbidity and mortality in patients undergoing hematopoietic stem cell transplantation (HSCT). We evaluated high-dose acyclovir (HDACV) and pre-emptive ganciclovir to prevent CMV disease in 76 children who underwent peripheral blood stem cell transplantation (PBSCT) and were at risk for CMV reactivation and disease (both recipient and donor seropositive) from May 1998 to April 2003. All received HDACV from day -9 to 6 months post transplant in conjunction with weekly CMV pp65 antigenemia monitoring. The incidence of antigenemia in this cohort was 19.7%, at a median of 22 days post-PBSCT. The frequencies were 26.4 and 4.4% in allogeneic and autologous groups, respectively (P=0.03). Patients with nonmalignant disease had higher CMV antigenemia than those with malignant disease (30.8 vs 8.1%, P=0.02). Age at PBSCT, sex, graft-versus-host disease (GVHD) prophylaxis regimen and presence of acute GVHD did not affect the risk of CMV antigenemia. None of the patients who had positive pp65 antigenemia developed CMV disease during the study period. We conclude that pp65 antigenemia-guided HDACV and pre-emptive ganciclovir may prevent CMV disease in children undergoing PBSCT.