Massive gastrointestinal hemorrhage sometimes occurs in Crohn's disease. To examine the possible role of acquired disorders of hemostasis contributing to such events, several laboratory indicators of hemostasis (APT time, Normotest, platelet count, bleeding time, fibrinogen, factor VIII activity, vWF:Ag, factor X, factor XIII, antithrombin III, flbrinopeptide A, and Bβ(15–42)) were studied in 10 patients with active Crohn's disease (Crohn's disease activity index (CDAI) >150) and 10 patients with quiescent disease (CDAI < 150). Marked thrombocytosis was seen in three patients with active disease. Factor VIII activity and fibrinogen levels were significantly elevated in patients with Crohn's disease (p < 0.001), and the factor VIII activity levels were significantly higher (p < 0.05) in the patients with active disease than in those with quiescent disease. Factor XIII levels were significantly lower (p < 0.02) in patients with active disease. Three of the patients with active disease had factor XIII levels below the lower reference limit. The two patients with the lowest levels had hemorrhagic diarrhea and spontaneous bleeding from the rectal mucosa. Fibrinopeptide A and Bβ(15–42) levels were significantly elevated in both groups. The other coagulation analyses were essentially normal in both patient groups. The results suggest that factor XIII deficiency acquired through gastrointestinal leakage may contribute to gastrointestinal hemorrhage in some patients with active Crohn's disease.