The objective of this study was to determine the immune responses to candidate viral triggers of multiple sclerosis in patients and healthy siblings raised in the same family household. Virus antigen-specific IgG responses to Epstein-Barr virus-derived gene products as well as to human herpersvirus-6, human cytomegalovirus, and measles virus were evaluated in 25 multiple sclerosis patients and compared with 49 healthy full-siblings. IgG responses to the latent Epstein-Barr virus-encoded nuclear antigen-1 (EBNA1) were selectively increased in individuals with multiple sclerosis compared with their unaffected siblings. We conclude that elevated IgG responses towards EBNA1 are associated with the development of multiple sclerosis.