Human herpesvirus 6 (HHV-6) infection is common in the general population. Primary infection occurs mainly in childhood, after which the virus establishes latency in the host and can be reactivated in immunocompromised individuals such as recipients of a solid organ or bone marrow transplant (BMT). HHV-6 infection occurs in 38--60% of BMT recipients and in 14--82% of solid organ recipients, usually 2--4 weeks post-transplantation. Two HHV-6 variants are recognized (HHV-6A and HHV-6B) and transplant patients are infected almost exclusively with variant B. Following BMT, the clinical diseases most commonly associated with HHV-6 infection are bone marrow suppression, interstitial pneumonitis, encephalitis and graft versus host disease. It has also been suggested that HHV-6 infection may be associated with fever and bone marrow suppression following liver transplantation and may contribute to rejection episodes after kidney transplantation. Infection with HHV-6 induces marked immunodepression, and an association both with cytomegalovirus disease and with fungal infection in transplant recipients has been reported. A direct causal relationship between HHV-6 infection and disease following transplantation, however, has yet to be proven conclusively.