HCMV DNA was retrospectively quantitated in the early post-transplant period in 36 paediatric bone marrow transplant (BMT) recipients prospectively monitored for human cytomegalovirus (HCMV) infection on the basis of antigenaemia and viraemia assays. Viral DNA was quantitated in peripheral blood leucocytes (PBL) by PCR using an internal control of amplification and a series of external standards. Densitometric analysis of hybridization results obtained on PCR products enabled construction of a standard curve from which DNA amounts of clinical samples, expressed in terms of genome equivalents (GE), were interpolated. Of the 36 BMT recipients, three had clinically symptomatic HCMV infection with mean peak levels of viral DNA > 5000 GE (antigenaemia and viraemia mean peak levels were 873 and 35, respectively), whereas 19 with HCMV reactivation were asymptomatic (five of them had abortive HCMV infection) showing mean peak DNA levels of 131 GE (and of 6.8 and 1.3 for antigenaemia and viraemia, respectively) (P < or = 0.01). Single or multiple courses of pre-emptive therapy with ganciclovir or foscarnet were given to 14/19 asymptomatic children in whom antigenaemia levels were > 2 or lower yet persisting. Overall, in the 14 asymptomatic treated patients the mean antigenaemia level was 9.3 (range 1-22), and the mean DNA level was 184.6 (range 20-710) GE. Antiviral drugs were also administered to the three symptomatic patients who, due to late diagnosis of HCMV infection, escaped preemptive therapy. Antiviral treatment caused marked decrease or disappearance of viral DNA, antigenaemia and viraemia in both symptomatic and asymptomatic patients. In conclusion, our study suggests that: (i) starting therapy in the presence of a mean antigenaemia level of 9.3 (range 1-22) corresponding to a mean DNA level of 184.6 (range 20-710) GE avoided occurrence of any major HCMV-related clinical complication; (ii) clinical symptoms were associated with antigenaemia levels > 100 and DNA levels > 1000 GE; (iii) the effect of antiviral treatment could be more carefully monitored by quantitation of viral DNA.