Objectives: In recent years, both the prevalence of obesity and the incidence of RA have been rising. Our aim was to assess the association between overweight or obesity and rheumatoid arthritis (RA). Design: Patients who were diagnosed with RA were compared with population-based controls, matched for age and sex (by a ratio of 1:5). Body measurements and smoking status were collected from medical records. Body mass index was classified in WHO categories of underweight, normal, overweight and obese (<18.5, 18.5-<25, 25-<30, ≥30 kg/m2). χ2 and t-tests and logistic regression models were used to compare the study groups and to assess the association between obesity and RA. Setting: A cross-sectional analysis performed utilizing the database of Clalit Health Services, the largest healthcare provider organisation in Israel. Data were collected from the beginning of computerised database usage (around year 2000) until 2015. Participants: CHS covers over 4.4 million enrollees, of which all RA patients and matched controls were selected. Main outcome measures: Proportion of obesity (BMI≥30.0 kg/m2) among RA patients and controls. Results: The study included 11 406 patients with RA and 54 701 controls. The proportion of obese subjects among RA patients was higher in comparison with controls, (33.4% vs 31.6%, respectively). In multivariate regression model, smoking and obesity were found to be associated with RA, whereas male gender was found as inversely related to RA. Conclusions: Our findings demonstrate that obesity is significantly associated with RA. This finding underlines the role that obesity plays in inflammation and autoimmune conditions.