Background: Platelets play an important role in hemostatic and inflammatory responses. To evaluate any potential enhancement of platelet activity in patients with inflammatory bowel disease (IBD), we measured the platelet aggregation responses to various stimuli. Methods: Twenty-two healthy controls, 24 patients with ulcerative colitis (UC) and 25 patients with Crohn’s Disease (CD) were studied. The aggregation responses induced by three agonists (epinephrine, collagen, and ADP)were measured by an 8- channel aggregometer. The platelet-derived microparticles (PDMP) levels were measured by an enzyme-linked immunosorbent assay. Results: Twenty-one out of the 22 healthy controls did not respond to epinephrine (0.1 μ g/ml), collagen (0.2 μ g/ml), or ADP (1.0 μ M). Eight out of the 12 active UC patients were sensitive to all agonists, and 4 patients showed increased sensitivity to epinephrine/collagen or epinephrine/ADP. Three out of the 12 inactive UC patients were normal, but 9 of these patients showed increased sensitivity, mainly to epinephrine. Ten out of the 12 active CD patients were sensitive to all agonists, and 2 active CD patients were sensitive to epinephrine/ collagen or epinephrine/ADP. Eight out of the 13 inactive CD patients were sensitive to two or all agonists. Even after remission, almost all of the UC and CD patients showed some increased sensitivity to the agonists. The platelet number and the plasma PDMP levels were significantly higher in the active IBD patients than in the control group. Conclusions: Platelet aggregation responses are enhanced in IBD, even in inactive-phase patients. This increased sensitivity of the platelets may play an important role in the pathophysiology of IBD. Background: Platelets play an important role in hemostatic and inflammatory responses. To evaluate any potential enhancement of platelet activity in patients with inflammatory bowel disease (IBD), we measured the platelet aggregation responses to various stimuli. Methods: Twenty-two healthy controls, 24 The combination responses induced by three agonists (epinephrine, collagen, and ADP) were measured by an 8- channel aggregometer. The platelet-derived microparticles (PDMP Results: Twenty-one out of the 22 healthy controls did not respond to epinephrine (0.1 μg / ml), collagen (0.2 μg / ml), or ADP (1.0 μ M out of 12 active UC patients were sensitive to all agonists, and all 4 patients showed increased sensitivity to epinephrine / collagen or epinephrine / ADP. Three out of the 12 inactive UC patients were normal, but 9 of these ten out of the 12 active CD patients were sensitive to all agonists, and 2 active CD patients were sensitive to epinephrine / collagen or epinephrine / ADP. Eight out of the 13 inactive CD patients were sensitive to Both or all agonists. Even after remission, almost all of the UC and CD patients showed some increased sensitivity to the agonists. The platelet number and the plasma PDMP levels were significantly higher in the active IBD patients than in the control group. aggregation reactions are enhanced in IBD, even in inactive-phase patients. This increased sensitivity of the platelets may play an important role in the pathophysiology of IBD.