Human herpesvirus-6 (HHV-6), human herpesvirus-7 (HHV-7), Epstein-Barr virus (EBV), and human cytomegalovirus (CMV) DNA were repeatedly assayed in peripheral blood leukocytes from 37 allogeneic bone marrow transplant (BMT) patients by polymerase chain reaction. Before BMT, HHV-6 DNA was detected in 8 (22%) patients. HHV-7, EBV, and CMV DNA were detected in 21 (57%), 10 (27%), and 1 (3%) patient, respectively. After BMT, HHV-6 DNA was detected in 26 (70%), HHV-7 in 21 (57%), EBV in 28 (76%), and CMV in 21 (57%) patients. Thirty-two (87%) patients were positive with more than one virus. HHV-6, HHV-7, and EBV DNA were found earlier than CMV DNA in most patients after BMT. The proportions of HHV-6-positive samples during the first 3 months after BMT were higher in the patients with either delayed granulocyte engraftment (P = .04, Fisher's exact test) or delayed platelet engraftment (P = .001, Fisher's exact test). The HHV-6 DNA in samples from the patients with delayed engraftment was confirmed to be variant B. The detection of any lymphotropic herpesvirus was not related to the development of acute graft-versus-host disease (aGVHD). High-dose acyclovir (ACV) prophylaxis significantly (P < .01) reduced the proportion of HHV-6-positive samples and tended to lower HHV-6 DNA levels (P = .06). Our data indicate that HHV-6 variant B can inhibit marrow engraftment and that high-dose ACV may be beneficial to engraftment after BMT by preventing HHV-6 reactivation. No relation between the proportions of HHV-7-, EBV-, and CMV-positive samples in the first 3 months and engraftment or aGVHD was found.