In our all patients, the antibodies to HHV-6 and -7 were positive before BMT. HHV -6 and -7 DNA were sometimes detected after BMT, and HHV-6 infection after BMT caused fever, interstitial peumonitis, diarrhea, and myelosuppression. However, HHV -7 didn't induce any clinical symptoms. For the diagnosis of the HHV-6 or -7 infection, we used the virus isolation, semiquantitative PCR, and 4-hold elevation of the antibodies to HHV-6 or -7. Ganciclovir, foscarnet, high dose gamma-globulin and high dose acyclovir were useful for the treatment of HHV-6 infection after BMT. HHV-6 is an important agent for the fever of unknown origin, interstitial peumonitis, diarrhea, and myelosuppression after BMT.