Adipose tissue possesses crucial immunological features in the development of comorbidities of obesity. Complex interactions between the metabolic system and cells of the immune system are the cause but are not fully understood. Autoimmune diseases might therefore be influenced by obesity. Epidemiological studies provide evidence of only a modest association between obesity and the development of rheumatoid arthritis and psoriatic arthritis. In obesity the course of inflammatory rheumatic diseases is aggravated, as subjectively measured by activity parameters; however, laboratory parameters show no differences and radiographic examinations reveal less bone destruction in obesity. Furthermore, there are indications for a poorer response to treatment especially with infliximab in patients with an increased body mass index (BMI). Weight loss has a positive effect on the course of joint diseases. Due to deficits in the currently available studies there is insufficient evidence to recommend individualization of treatment decisions based on the BMI.