One mechanism underlying the link between multiple sclerosis (MS) and Epstein-Barr virus (EBV) might be a direct CNS infection. Viral CNS infections cause elevated antibody indices (AIs). Elevated EBV AIs were found in MS; however, patients with MS frequently show a polyspecific intrathecal immune response with elevated antiviral AIs. To discriminate whether elevated EBV AIs indicate a virus-driven or a polyspecific intrathecal immune response, we determined the intrathecal fraction of anti-EBV antibodies.