An important role has been ascribed to plasma factor XIII (FXIII) in inflammation and wound healing. FXIII is necessary for fibrin stabilization and interacts with connective tissue and adhesive proteins and cells. In a prospective study, FXIII activity and parameters of coagulation, fibrinolysis and inflammation, were determined in patients with ulcerative colitis (UC; 13 active, 22 moderate) and Crohn's disease (CD; 36 active, 45 moderate). FXIII levels were lower in active than in moderate UC and CD, and were < 70% of normal values in 7/13 patients with active UC, and in 7/36 patients with active CD, although the median values did not fall below the normal range. FXIII was somewhat higher in active UC patients responding to therapy. The FXIII levels were widely scattered, and low values appear to be due to greatly enhanced turnover. A correlation between FXIII and the systemic levels of markers of activation of haemostasis and inflammation was lacking, but there was a correlation with the extent of bowel involvement. FXIII levels were lower in the patients with involvement beyond the sigmoid colon in CU (p = 0.0045), and both small and large bowel segments in CD (p = 0.0223) patients. This points to local consumption and/or loss of FXIII within the inflamed tissue, and provides an argument for FXIII substitution in the treatment of acute episodes of inflammatory bowel diseases.