In this issue of Arthritis Care & Research, we present the eleventh in a series of themed issues that are highly relevant to rheumatologic practice and clinical study of the rheumatic diseases. These themed issues, which appear once or twice a year, are designed to highlight state-of-the-art information in the field of rheumatology. The topic for this themed issue is obesity and fat mass in relation to a number of specific rheumatic diseases. Notably, rheumatoid arthritis and osteoarthritis are represented by seven articles and four articles, respectively. Other entities of interest such as gout, psoriatic arthritis, and polyarthritis are also covered. A solicitation for manuscripts for the next theme, concerning mobility issues in the rheumatic diseases, both pediatric and adult, has been announced. The manuscripts submitted for the theme issues navigate the same peer-review process of Arthritis Care & Research as do all scientific manuscripts, and therefore meet the same rigorous standards as articles in this or any other issue.

Western societies have renewed the focus on the obesity epidemic in the general population. The public health impact of obesity in the rheumatic diseases, whether in the causal pathway, related to disease symptomatology, or as a result of treatment, becomes even more essential to describe and investigate. The long-term consequences of rheumatic disease alone have major impact, but in synergy with increased fat mass or obesity, the burden is even greater. Our previous theme issue focused upon muscle, bone, and sarcopenia. One anticipates a better understanding of these body composition components along with fat mass and obesity due to the important clinical studies involving crucial groups of rheumatic disease patients that are encouraging new ideas and a more integrated knowledge of these effects.

The role of fat mass in many rheumatic diseases is of great concern to clinicians as well as patients. Articles in this issue reported obesity effects upon the severity of rheumatoid arthritis and comorbidities. Other topics chosen for this theme issue focus on changes in body mass index related to glucocorticoid use in pediatric patients and changes in treatment responses. Also presented for the readership are sex differences in the assessment of obesity, as well as measurement of intermuscular fat volume in relation to physical performance. Longitudinal changes in disease outcomes in relation to obesity and body weight were presented in several articles.

The call for papers for this theme issue resulted in more than 45 submissions; from among these, we are proud to publish 18 original articles covering important aspects of the influences of obesity upon the rheumatic diseases. These papers are intended to spark intellectual curiosity and generate interest in a more systematic evaluation of the role that obesity plays in the pathophysiology of, the interactions with, and outcomes in the rheumatic diseases as our understanding of obesity and its synergies in the rheumatic diseases comes under greater examination in the years ahead.