A new observation of inherited factor XIII deficiency is described. The patient had presented in the past with an umbilical haemorrhage, a cerebral haemorrhage, multiple haematomas with delayed onset after minimal trauma, and episodes of spontaneous haemarthrosis. The biological diagnosis was made at the age of 7 years. The child had undetectable transamidating activity and factor XIII a chains, while the level of b chains was reduced. The values observed in the parents were intermediate between those of the patient and those of normal plasma. Heated factor VIII concentrates were found to contain only low amounts of factor XIII, and were thus unsuitable for the prophylactic therapy of this rare disease. The patient was successfully treated with monthly injections of a factor XIII concentrate.