Cytomegalovirus (CMV) DNAemia was detected by PCR in 30/125 (24%) consecutive paediatric patients undergoing allogeneic stem cell transplantation. All patients with CMV DNAemia received pre-emptive ganciclovir until two consecutive negative results were obtained. CMV-IgG-positive patients (R+) had a significantly increased risk of DNAemia as compared to CMV-IgG-negative (R-) patients (62% vs 8%) P<0.0001. The incidence of DNAemia was 71% (10/14) in R+ transplanted from seronegative donors (D-) compared to 54% (13/32) in those transplanted from seropositive donors (D+). Of 30 (40%) children with DNAemia, 12 developed CMV disease despite pre-emptive treatment. The overall incidence of disease was 0% (0/59) for R-/D-, 9% (3/23) for R+/D+, 7% (2/29) for R-/D+ and 57% (8/14) for R+/D-. In patients with DNAemia, 4/20 (20%) patients with D+ and 8/10 (80%) with D- became symptomatic. In the multivariate analysis of both groups, patients at risk (R+ and/or D+) and patients with DNAemia, a negative donor serostatus was the only factor associated with a significantly increased incidence of disease. Seven of 9 patients with lethal CMV disease had received CMV-IgG-negative grafts. The data suggest that in CMV seropositive recipients donor CMV seropositivity is associated with a reduced incidence of CMV disease and a favourable outcome following pre-emptive treatment.