Human herpesvirus-6 (HHV-6) is the most recently discovered herpesvirus. It shares several characteristic properties with the other herpesviruses; the genome is large, consisting of approximately 170,000 base pairs and potentially containing more than 70 genes. The virus also probably forms latent infections in the host. It is widely distributed in the population, and seropositivity reaches 60-90% by the age of two. HHV-6 is associated with the childhood disease exanthema subitum, and probably also causes heterophile antibody-negative mononucleosis. Our present knowledge about the virus is reviewed.