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, fibrinopeptide A, and B beta(15-42] were studied in 10 patients with active Crohn's disease (Crohn's disease activity index (CDAI) greater than 150) and 10 patients with quiescent disease (CDAI less than 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 less than 0.001), and the factor VIII activity levels were significantly higher (p less than 0.05) in the patients with active disease than in those with quiescent disease. Factor XIII levels were significantly lower (p less than 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 B beta(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.