FXIII deficiency is known as one of the rarest blood coagulation disorders. In this study, the phenotypic and in part genotypic data of 104 FXIII-deficient patients recorded from 1993 - 2005 are presented. The most common bleeding symptoms were subcutaneous bleeding (57%) followed by delayed umbilical cord bleeding (56%), muscle hematoma (49%), hemorrhage after surgery (40%), hemarthrosis (36%), and intracerebral bleeding (34%). Prophylactic treatment was initiated in about 70% of all patients. FXIII-B subunit-deficient patients had a milder phenotype than patients with FXIII-A subunit deficiency. The most frequent mutation affecting the F13A gene was a splice site mutation in intron 5 (IVS5-1G>A). This mutation was found in eight (17%) of 46 analyzed families. The haplotype analysis of patients carrying the IVS5-1A allele was consistent with a founder effect. The international registry (http://www.f13-database.de) will provide clinicians and scientists working on FXIII deficiency with a helpful tool to improve patient care and direct future studies towards better understanding and treatment of the disease.