Patients with inflammatory bowel disease (IBD) are at higher risk of venous thromboembolism and coronary artery disease despite having a lower burden of traditional risk factors. Platelets from IBD patients release more soluble CD40 ligand (CD40L), and this has been implicated in IBD platelet hyper-activation. We here measured the urinary F2-isoprostane 8-iso-prostaglandin (PG)2α (8-iso-PGF2α), urinary 11-dehydro-thromboxane (TX) B2 (11-dehydro-TXB2) and plasma CD40L in IBD patients, and explored the in vitro action of anti-tumour necrosis factor (TNF)-α antibody infliximab on IBD differentiating megakaryocytes. Urinary and blood samples were collected from 124 IBD patients and 37 healthy subjects. Thirteen IBD patients were also evaluated before and after 6-week infliximab treatment. The in vitro effect of infliximab on patient-derived megakaryocytes was evaluated by immunoflorescence microscopy and by flow cytometry. IBD patients had significantly (p<0.0001) higher urinary 8-iso-PGF2α and 11-dehydro-TXB2 as well as plasma CD40L levels than controls, with active IBD patients displaying higher urinary and plasma values when compared to inactive patients in remission. A 6-week treatment with infliximab was associated with a significant reduction of the urinary excretion of 8-iso-PGF2α and 11-dehydro-TXB2 (p=0.008) and plasma CD40L (p=0.001). Infliximab induced significantly rescued pro-platelet formation by megakaryocytes derived from IBD patients but not from healthy controls. Our findings provide evidence for enhanced in vivo TX-dependent platelet activation and lipid peroxidation in IBD patients. Anti-TNF-α therapy with infliximab down-regulates in vivo isoprostane generation and TX biosynthesis in responder IBD patients. Further studies are needed to clarify the implication of infliximab induced-proplatelet formation from IBD megakaryocytes.