Severe homozygous factor XIII deficiency was first described in Switzerland, in 1961. At present 14 patients are known here. Nine are of Swiss origin, the others are immigrants from eastern Europe. A 27-year-old woman with many haemorrhages during childhood immigrated to Switzerland and went through four episodes of haemorrhagic corpus luteum cyst rupture with life-threatening blood loss into the abdomen and three haemorrhages into the retroperitoneal muscles causing sensomotoric palsies, before the diagnosis was established. A monthly prophylactic replacement therapy of 500 IE factor XIII concentrate was started. Since then no signs of haemorrhage occurred. For the last trimester of pregnancy treatment intervals were shortened and dosage increased. Haemorrhage from the umbilical cord for weeks, subcutaneous haematomas, intracranial haemorrhage, muscle haemorrhage and wound bleeding with impaired wound healing as well as tendency to marked scar formation are characteristic for severe homozygous factor XIII deficiency. Without replacement therapy women suffer from obligate abortion. Diagnosis is made by the solubility of fibrin clots in urea (5 mol/l) or monochloroacetic acid (1-2%). For confirmation and monitoring of replacement therapy a quantitative incorporation assay is used. Replacement therapy is necessary in case of haemorrhage, injury, and surgery. Because of the high risk of intracranial haemorrhage prophylaxis is strongly recommended.