Inflammatory bowel disease (IBD) is known to be associated with a thrombotic tendency, which is often attributed to thrombocytosis, elevated fibrinogen, or decreased antithrombin III. We prospectively studied eight patients with IBD, seven of whom had little or no disease activity, to determine if they had any laboratory abnormality known to be associated with an increased risk of thrombosis. Abnormalities in fibrinolysis were noted in five patients: four with high plasminogen activator inhibitor levels and one with poor release of tissue plasminogen activator following venous occlusion. Circulating immune complexes were present in the sera of five patients. Fibrinogen was mildly elevated in one patient, and two patients had mild thrombocytosis. Decreased levels of antithrombin III, protein C, or protein S were not observed. There appears to be a high incidence of abnormalities in fibrinolysis in inactive IBD, which may contribute to the high frequency of thrombosis seen in IBD. The presence of circulating immune complexes may contribute to vascular injury and thrombosis.