In a recent study we summarized retrospective results of the determination of HHV infection status in allogeneic stem cell transplant recipients. As infections with HHV-6 are rarely accompanied by clinical symptoms, our first aim was to compare HHV-6 infection status, measured as viral DNA presence in the blood in the post-transplantation period and patients' anti-HHV-6 serological status by detection of IgG and IgM antibodies. This allowed us to determine both the presence of active infection and whether a recent infection was a result of primary contact with the virus or the reactivation of latent virus, which should be valuable information in terms of comparing HHV-6 infection status and clinical status of an infected patient.