INTRODUCTION: Patients with Crohn's disease (CD) and ulcerative colitis (UC) may lose weight during periods of active disease and may gain weight when inflammation heals. Studies have hypothesized an association between antitumor necrosis factor-alpha (anti-TNF-α) and unintended weight gain during maintenance therapy, and this association has not been previously clarified. METHODS: In a nationwide observational study based on Danish national health registries, we included patients who initiated therapy with infliximab and followed changes in weight during induction therapy (0-90 days) and maintenance therapy (91-270 days). The association between the use of infliximab and weight gain was analyzed by a multilevel mixed-effects linear regression model. RESULTS: Among 851 patients with CD and UC who initiated infliximab therapy, long-term weight gain was not observed during maintenance therapy in most of the patients. Women with CD who were underweight at the initiation of therapy had an average weight gain of 7.5 kg. Men and women with CD and UC with normal or increased body mass index had an average weight gain of <2 kg during maintenance therapy. Underweight men with CD and UC gained 2.9 kg (95% confidence interval 2.1-3.6) and 2.9 kg (95% confidence interval 1.9-3.9), respectively, in the first 90 days, although neither group had statistically significant weight gain in the maintenance period. Less than 3% of the patients had weight gain greater than 10% of their baseline body weight during the study period. DISCUSSION: Weight gain among patients treated with anti-TNF-α therapies is unlikely to be due to an effect from anti-TNF-α therapy.