Objective. To evaluate changes in body mass index (BMI) among cohorts of children with juvenile idiopathic arthritis (JIA) with and without tumor necrosis factor (TNF) inhibitor therapy. Methods. We performed a retrospective chart review of children with JIA who newly initiated TNF inhibitor therapy and had at least 1 year of subsequent followup (TNF cohort). We also included children with JIA and at least 1 year of followup without any TNF inhibitor therapy (comparator cohort). Children with systemic arthritis were excluded. Age and sex specific BMI z-scores and their corresponding categories (normal, overweight, obese) were determined. We compared changes from a baseline visit to the last followup visit using t-test for BMI z-scores and chi square and Kruskal-Wallis tests for BMI categories. Results. The TNF cohort had 167 patients; the comparator cohort had 37. The median study followup was 2.8 and 2.2 years, respectively. The cohorts had similar age, sex, race, weight, and height distributions. The TNF cohort had a statistically significant increase in BMI z-score from baseline (+0.15; p = 0.02) that was not significantly different from the increase (+0.09) observed in the comparator cohort (p = 0.5). There was no significant change in the proportions of overweight and obese children in the TNF cohort compared to baseline (p = 0.6) or compared to the change in the comparator cohort (p = 0.2). Conclusion. Over more than 2 years of followup, we did not observe a significant increase in BMI among children with JIA receiving TNF inhibitor compared to those not receiving it. The Journal of Rheumatology Copyright © 2014. All rights reserved.