Since their appearance more than a decade ago, anti-tumor necrosis factor (TNF) inhibitors have demonstrated beneficial activity in the treatment of inflammatory bowel diseases (IBD). However, more than one-third of patients present primary resistance, and one more third become resistant over time. One of the main factors associated with loss of response is the immunogenicity of anti-TNF biologics leading to the production of antibodies targetting the TNF inhibitor, namely anti-drug antibodies (ADAbs), that accelerate drug elimination from the serum and decrease its therapeutic activity. In this study the investigators propose a medico-economic evaluation of the mesurement of anti-TNF agents and anti-drug antibodies serum concentrations in the management of patients with inflammatory bowel disease treated with anti-TNFalpha inhibitors. 280 patients with Crohn's disease (CD) or ulcerative colitis (UC) will be included and randomized in 2 groups with or without drug and ADAbs monitoring. In the monitored group, in case of loss of response, the clinician will use biological informations to adapt the treatment following a simple treatment algorithm. In the unmonitored group, drug and ADAbs measurements will not be transmitted to the clinician. Clinical and economical benefits of the biological monitoring will be evaluated after a follow-up period of two years.