A 33 years old female with ulcerative colitis was admitted with an acute exacerbation of the disease characterised by haematochezia, diarrhoea (10 stools/die) and anaemia (haemoglobin 6.5 g/dl). Therapy with 5-ASA and corticosteroids for six weeks failed to decrease the activity of the disease. Since deficiency of coagulation factor XIII (activity 60%, subunit A 56%) was present, in addition, concentrates of factor XIII (Fibrogammin HS, Behring, F.R.G.) 1250 U/die were given for ten days. The substitution resulted in an immediate increase of reduced F XIII activity (164%) and F XIII subunit A (333%) as well as in a marked improvement of symptoms.