Patients with active ulcerative colitis have decreased levels of factor XIII (FXIII) activity, which is important for woundhealing. Recent uncontrolled studies claimed a beneficial effect of Factor XIII on clinical symptoms of ulcerative colitis, in particular intestinal bleeding. The objective of this trial was to evaluate the benefits of additional FXIII treatment in steroid-refractory patients with ulcerative colitis in a prospective, double blind, placebo-controlled study. A total of 28 patients were enrolled between October 1994 and January 1997. Primary objective of this study was the time until cessation of visible intestinal bleeding with 14 days after the start of treatment. Patients were treated for ten days either by i.v. application of FXIII concentrate or by placebo. The analysis of the primary efficacy criterion, cessation of intestinal bleeding, by a planned interim analysis showed no significant differences between the treatment groups (p = 0.8). This resulted in the termination of the study. The same applied to the CAI score. No patient in both treatment groups reached remission according to the colo-/-sigmoidoscopy score. Due to the high number of patients (16 of 28) who had to be excluded from the per-protocol analysis (e.g. changes to the concomitant medication) only the intention-to-treat population was analyzed. Overall the study showed no beneficial effect of additional FXIII treatment on active steroid-refractory ulcerative colitis. These results do not confirm previous open label studies which had reported a significant improvement of clinical symptoms.