BackgroundEarlier studies have demonstrated that smoking and genetic risk factors interact in providing an increased risk of rheumatoid arthritis (RA). Less is known on how smoking contributes to RA in the context of genetic variability, and what proportion of RA may be caused by smoking.ObjectivesTo determine the association between the amount of smoking and risk of RA in the context of different HLA-DRB1 shared epitope (SE) alleles, and to estimate proportions of RA cases attributed to smoking.Design, Setting and ParticipantsData from the Swedish Epidemiological Investigation of Rheumatoid Arthritis (EIRA) case–control study encompassing 1204 cases and 871 controls were analysed.Main OutcomeMeasure Estimated OR to develop RA and excess fraction of cases attributable to smoking according to the amount of smoking and genotype.ResultsSmoking was estimated to be responsible for 35% of anticitrullinated protein/peptide antibody (ACPA)-positive cases. For each HLA-DRB1 SE genotype, smoking was dose-dependently associated with an increased risk of ACPA-positive RA (p trend <0.001). In individuals carrying two copies of the HLA-DRB1 SE, 55% of ACPA-positive RA was attributable to smoking.ConclusionsSmoking is a preventable risk factor for RA. The increased risk due to smoking is dependent on the amount of smoking and genotype.