Human herpesvirus 7 (HHV-7) is the least studied β-herpesvirus in transplant settings. This prospective study examined the activity of HHV-7 during the first 12 weeks post-stem cell transplant in 59 paediatric patients. The presence of HHV-7, human cytomegalovirus (HCMV) and human herpesvirus 6 (HHV-6) in blood was monitored weekly by a multiplex nested polymerase chain reaction. Overall, 33 (55.9%) patients had one or more surveillance blood sample(s) positive for HHV-7. In contrast to HCMV and HHV-6, no obvious peak time of reactivation was observed for HHV-7. The occurrence of HHV-7 DNAaemia showed a significant negative association with HHV-6 (P = 0.022), but with no association with HCMV. A significant higher positive rate for HHV-7 was found in autologous versus allogeneic (P = 0.002), and in peripheral blood versus umbilical cord/marrow (P < 0.001) transplant. Acyclovir had no effect, whereas ganciclovir was associated with a lower rate of HHV-7 reactivation (P = 0.009). One patient died of HHV-7 associated brain stem encephalitis. The administration of colony stimulating factor, occurrence of acute graft versus host disease, time to neutrophil and platelet engraftment showed no significant association with the occurrence of HHV-7 DNAaemia. J. Med. Virol. 72:668–674, 2004. © 2004 Wiley-Liss, Inc.