Pathogenetic roles of human herpesvirus (HHV)-6 in lymphoproliferative diseases have been of continued interest. Many molecular studies have tried to establish a pathogenic role for HHV-6 in lymphoid malignancies. However, whether HHV-6 plays a role in these pathologies remains unclear, as positive polymerase chain reaction results for HHV-6 in those studies may reflect latent infection or reactivation rather than presence of HHV-6 in neoplastic cells. A small number of studies have investigated HHV-6 antigen expression in pathologic specimens. As a result, the lack of HHV-6 antigen expression on neoplastic cells argues against any major pathogenic role of HHV-6. The role of HHV-6 in childhood acute lymphoblastic leukemia (ALL) has also been of interest but remains controversial, with 2 studies documenting higher levels of HHV-6 antibody in ALL patients, and another 2 large-scale studies finding no significant differences in HHV-6 seroprevalences between ALL patients and controls. Alternatively, HHV-6 is increasingly recognized as an important opportunistic pathogen. HHV-6 reactivation is common among recipients of allogeneic stem cell transplantation (SCT), and is linked to various clinical manifestations. In particular, HHV-6 encephalitis appears to be significant, life-threatening complication. Most HHV-6 encephalitis develops in patients receiving transplant from an unrelated donor, particularly cord blood, typically around the time of engraftment. Symptoms are characterized by short-term memory loss and seizures. Magnetic resonance imaging typically shows limbic encephalitis. Prognosis for HHV-6 encephalitis is poor, but appropriate prophylactic measures have not been established. Establishment of preventive strategies against HHV-6 encephalitis represents an important challenge for physicians involved with SCT. [J Clin Exp Hematopathol 49(2) : 57-67, 2009]