OBJECTIVES: Paediatric and adult inflammatory bowel disease (pIBD, aIBD) patients may lose response to anti-tumour necrosis factor (TNF) treatment within the first year. Adult-extrapolated weight-based dosing is incorrect in children, due to age-related pharmacokinetic differences. We investigated biomarkers for initial and maintenance of response to infliximab (IFX) or adalimumab (ADA), comparing pIBD and aIBD patients. METHODS: In this prospective, observational study, pIBD (n = 24) and aIBD (n = 21) patients were included when initiating anti-TNF. Escalation from standard dosing and continued anti-TNF at 12 and 18 months were assessed. Biomarkers included clinical laboratory parameters, faecal calprotectin (FCP) and IFX trough levels (TLs). Plasma proteomics was performed in pIBD. RESULTS: During our study, treatment escalation (in clinical loss of response) occurred more common in pIBD versus aIBD (p = 0.02). We established that IFX therapy escalation in pIBD patients was not due to low infliximab levels. We identified 9 pro-inflammatory proteins that were elevated in patients losing response. CONCLUSION: Anti-TNF exposure-response relationship may be different in pIBD versus aIBD. No biomarkers for maintained response were identified, but 9 inflammatory proteins were of interest as potential predictors for loss of response in pIBD.