A congenital FXIII A subunit deficiency was diagnosed in a male child because of umbilical bleeding at birth. Venous infusion was difficult and prophylactic FXIII infusion was delayed. At age 1, he suffered a spontaneous intracranial haemorrhage. Substitutive FXIII was initiated, and at age 12, no other significant bleeding event had occurred. His 5 years younger brother also bears the same FXIII deficiency. The younger brother's treatment was initiated at birth and never discontinued, and no bleeding occurred. Mutation gene analysis found a homozygous four bases insertion predicting a stop codon seven residues after PRO675. Antigen assay indicated that the mutant molecule is secreted. This case highlights the importance of prophylactic FXIII infusion.