By matching a cohort of 494 infectious mononucleosis (IM) cases with a multiple sclerosis (MS) register, 3 MS cases were retrieved. The interval between IM and MS was 12 years. This corresponds to a relative risk of 3.7 for MS to occur subsequent to IM (p = 0.05). This relationship between a manifestation of a relatively late Epstein-Barr virus infection and MS may indicate that a microbiologically shielded environment is important in the pathogenesis of MS.