BACKGROUND: The aim of the study was to investigate variations in anti-infliximab (IFX) antibody (Ab) levels and clinical implications thereof in patients with inflammatory bowel disease (IBD). METHODS: A retrospective, explorative, single-center study of patients with IBD who developed anti-IFX Ab and in whom anti-IFX Ab were reassessed. RESULTS: IFX was administered to 316 patients; anti-IFX Ab was determined in 180 patients and detected in 83 (46%). During ongoing IFX maintenance therapy, anti-IFX Ab disappeared at later reassessment in two-thirds of patients with clinical response after median 4 (3-5) infusions. In contrast, anti-IFX Ab persisted in all patients without clinical response. Anti-IFX Ab appeared pharmacologically active, as IFX levels were high when anti-IFX Ab disappeared (median 3.7 µg/mL, interquartile range [IQR] 2.8-5.5), while undetectable or low when anti-IFX Ab persisted (median 0 µg/mL, IQR 0-0). In 56 patients, anti-IFX Ab were assessed after IFX discontinuation. The proportion of patients with anti-IFX Ab gradually declined over time, with a few patients having anti-IFX Ab up to about 4 years after initial assessment. No variables were associated with anti-IFX Ab disappearance in multivariate analysis. CONCLUSIONS: Discontinuation of IFX is advisable in patients with inadequate response and repeat positive anti-IFX Ab measurements. Anti-IFX Ab can persist for years after discontinuation, which could impact efficacy and safety at retreatment. Continued IFX treatment may, however, be considered in patients with clinical response and a single positive anti-IFX Ab measurement, as anti-IFX Ab disappears in two-thirds of these during continued treatment. (Inflamm Bowel Dis 2012;).