Based on the description of a severe bleeding disorder in a young child a short overview on the genetics, the epidemiology, the pathophysiology, the clinical manifestations and the laboratory diagnosis of factor XIII deficiency is presented. The impressive clinical signs with bleeding starting in the neonatal period (prolonged bleeding from the umbilical cord), followed by severe, life-threatening episodes of intracranial hemorrhages should raise the clinical suspicion of FXIII deficiency. Difficulty of laboratory diagnosis is stressed. The importance of repeating initially negative screening tests and of performing a quantitative FXIII assay in the presence of strong clinical suspicion is strengthened. The diagnosis of factor XIII deficiency is difficult but has important therapeutic consequences: patients with documented severe deficiency should be put on regular substitution with factor XIII concentrates. Appropriately timed periodic infusions of such factor XIII concentrates enable patients to live normal lives, free from catastrophic bleeding episodes.