A large number of hemostasis measurements complications were performed in 20 patients with inflammatory bowel disease; whose changes have been associated with an increased risk of thromboembolic. Of the 20 patients, 9 (45%) had one or more changes suggestive of activation of the hemostatic system. Such changes were more frequently observed in patients with active disease. Two patients had changes in the fibrinolytic system (high plasminogen activator inhibitor-1 levels) and 2 in the natural anticoagulants (low antithrombin III activity). Nine patients had increased plasma fibrinogen levels and 6 patients had slightly increased levels of anticardiolipin antibodies. Most of the changes observed were not related to the type, location or activity of the disease. These data show that various biochemical abnormalities may be found in patients with inflammatory bowel disease, which may account for their increased risk of thrombosis, and suggest that multiple mechanisms may interact in determining such complications.