Obesity is characterised by low-grade inflammation and could potentially affect disease activity and severity in patients with rheumatoid arthritis (RA). Body mass index (BMI), body fat (BF), erythrocyte sedimentation rate, C-reactive protein, disease activity score 28, physical function (health assessment questionnaire) and presence of erosions and joint surgery were assessed in 294 (female=219) volunteers with established RA [age 63.3 (56.2-69.6); disease duration 13 (7-20) years]. Smoking status, rheumatoid factor and anti-cyclic citrullinated peptide positivity were also assessed. BMI and BF independently associated with disease characteristics. Compared to normal-weight patients, underweight and obese had higher C-reactive protein (p=0.046) and physical dysfunction (p=0.034). BMI or BF did not associate with presence of erosions or joint surgery. In patients with established RA, both very low and very high BMI and BF associate independently with increased disease activity and physical dysfunction; however, this does not seem to associate with presence of erosions or joint surgery. Further longitudinal studies are required to address this apparent dissociation.