It is unclear whether smoking interacts with different aspects of Epstein-Barr virus (EBV) infection with regard to multiple sclerosis (MS) risk. We aimed to investigate whether smoking acts synergistically with elevated EBNA-1 antibody levels or infectious mononucleosis (IM) history regarding MS risk. Two Swedish population-based case-control studies were used (6,340 cases and 6,219 matched controls). Subjects with different smoking, EBNA-1 and IM status were compared regarding MS risk, by calculating odds ratios (OR) with 95% confidence intervals (CI) employing logistic regression. Potential interaction on the additive scale was evaluated by calculating the attributable proportion due to interaction (AP). Current and past smokers had higher EBNA-1 antibody levels than never smokers (p < 0.0001). There was an additive interaction between current smoking and high EBNA-1 antibody levels (AP 0.3, 95% CI 0.2-0.4), but not between past smoking and high EBNA-1 antibody levels (AP 0.01, 95% CI − 0.1 to 0.1), with regard to MS risk. An interaction also occurred between current smoking and IM history (AP 0.2, 95% CI 0.004-0.4), but not between past smoking and IM history (AP − 0.06, 95% CI − 0.4 to 0.3). Current smoking increases EBNA-1 antibody levels and acts synergistically with both aspects of EBV infection to increase MS risk, indicating that there is at least one pathway to disease in which both risk factors are involved. Multiple sclerosis (MS) is an inflammatory disease of the central nervous system with underlying genetic and environmental factors. Smoking, high levels of EBNA-1 Immunoglobulin G (IgG), and infectious mononucleosis (IM) history have consistently been associated with increased MS risk 1-3. Previous studies on the interplay between smoking and aspects of EBV infection have used different study designs and statistical methods, and sample sizes have often been limited which may have contributed to conflicting results (Table 1). The first study observed a positive interaction on the multiplicative scale between smoking and elevated EBNA-1 IgG levels with regard to MS risk 4. Smoking was only observed to increase MS risk among subjects with high EBNA-1 IgG levels. These findings were not replicated in other studies 5-7. A recent study showed a trend towards a negative interaction among young adults and a trend towards a positive interaction among older subjects, as defined by age at EBV assessment, suggesting age may influence the interaction between smoking and EBNA-1 IgG levels 8. Results from previous studies on a possible interaction between smoking and IM history have also been conflicting. Two studies found no interaction between smoking and IM history 6,7 , whereas a recent study reported a negative interaction on the multiplicative scale regarding MS risk 9. Using two Swedish population-based case-control studies comprising 6,340 cases and 6,219 controls, we aimed to investigate the interplay between smoking and different aspects of EBV infection [elevated EBNA-1 IgG levels and infectious monon...