Mechanisms causing persistence and reactivation of measles virus in subacute sclerosing panencephalitis (SSPE) are unknown. Borna disease virus (BDV) frequently causes latent or persistent infection in the nervous system. We investigated a possible association of these viruses in SSPE. Although BDV seropositivity was similar in SSPE and control groups, SSPE patients with high antibodies to BDV had earlier and more rapid disease. The findings suggest that BDV might be involved in the course, but not in the etiopathogenesis, of SSPE.