The exact association between HHV-6 reactivation and mortality found in this study is not clear. Three of the patients (38%) with detectable HHV-6 DNA levels died during viremia shortly after transplantation, all due to coexisting pneumonia of unconfirmed etiology. Acyclovir in typical doses was used as an antiviral prophylaxis during this period, but without any visible clinical success. Studies in vitro have shown that HHV-6 DNA replication is inhibited by ganciclovir, foscarnet, and cidofovir, but not by acyclovir [1] .