Multiple sclerosis (MS) is one of the most common disabling neurological diseases affecting young adults. It is a chronic disease characterised by inflammation and demyelination. The aetiology of MS is still unknown, but involvement of viruses has been suspected for many years. Recently much interest has focused on human herpesvirus 6 (HHV-6), since the virus has been detected in MS plaques in the brain and patients with MS have been shown to have an aberrant immune response to HHV-6. Results from different studies are, however, conflicting and in the light of the long list of previous claims to have found the viral aetiology of MS it is necessary to interpret the HHV-6 findings with great caution. Possible mechanisms for virally induced demyelination and autoimmunity are discussed in this review, and the evidence for and against a role for HHV-6 in MS is summarised.