INTRODUCTION: Tumor necrosis factor alpha (TNF alpha) blockers such as infliximab (IFX) and adalimumab (ADA) had significantly changed the course of inflammatory diseases such as rheumatoid arthritis (RA), spondyloarthritis (SpA) and Crohn’s disease (CD). However, about 30% of patients do not respond to these treatments. This lack of response may be due to the formation of antibodies against these drugs (anti-drug antibodies: ADAbs). The aim of this study was to determine the prevalence ADAbs against IFX and ADA, and the trough serum concentration of IFX and ADA in RA, SpA or CD patients and to assess their impact on the therapeutic response. METHODS: A cross sectional, multi-centric study was conducted including patients with RA, SpA or CD treated with IFX or ADA as a first biotherapy for at least 6 months. ADAbs and trough levels were measured by an Enzyme Linked Immunosorbent assay (ELISA). RESULTS: 137 patients were included (37 RA, 53 SpA and 47 CD). ADAbs were positive in 40% of cases for IFX and 25% for ADA. They were positive in 39% of SpA, 35% of RA, and 21% of CD. The presence of ADAbs was inversely correlated to the trough levels of IFX and ADA during RA (p=0.01 and p<0.0001), SpA (p<0.01 and p<0.0001) and CD (p=0.001 and p=0.04). For all pathologies, the presence of ADAbs was not correlated with disease activity. CONCLUSION: In our study, the presence of ADAb and low trough levels seem to not affect the therapeutic response in patients on TNF alpha antagonists.