BackgroundEpstein-Barr virus (EBV) is thought to be a necessary causative agent in the development of multiple sclerosis (MS). Infectious mononucleosis (IM), which occurs up to 70% of adolescents and young adults with primary EBV infection, appears to be a further risk factor but few studies have been highly powered enough to explore this association by time since IM diagnosis.ObjectiveThe objective was to quantify the risk of MS in individuals with IM compared with the general population, with particular focus on time since IM diagnosis.MethodsIn this retrospective cohort study using English national Hospital Episode Statistics from 2003 to 2023, patients with a hospital diagnosis of IM were compared with the general population for MS incidence.ResultsMS incidence in patients with IM was nearly three times higher than the general population after multivariable adjustment (adjusted hazard ratio = 2.8, 95% confidence interval (CI = 2.3-3.4), driven by strong associations at long time intervals (>5 years) between IM diagnosis and subsequent MS diagnosis.ConclusionWhile EBV infection may be a prerequisite for MS, the disease process of IM (i.e. the body's defective immune response to primary EBV infection) seems to be, in addition, implicated over the long term.