Twenty two patients with exacerbation of inflammatory bowel disease (19 with Crohn's disease, 3 with ulcerative colitis) and thrombocytosis were tested for possible activation of the coagulation and platelet system. Fifteen patients had abnormal platelet function i.e. unphysiologically high sensitivity in vitro towards ADP 2 mumol/l aggregation induction. In 81.8% of the patients we found enhanced fibrinogen concentrations. In 22.7% of the patients thrombin-antithrombin III values exceeded the upper limit of the reference range, and in 68.2% of the patients the D-Dimer concentration exceeded the upper reference limit as a result of reactive fibrinolysis. The altered platelet count and function, and the increased levels of fibrinogen and thrombin-antithrombin III with reactive fibrinolysis activation indicate the presence of prethrombotic factors in patients with exacerbation of inflammatory bowel disease. The presence of enhanced fibrinolysis in these patients might have consequences for the therapeutic treatment.