Factor XIII deficiency is a congenital defect of the blood coagulation system. Activated factor XIII is the fibrin cross-linking enzyme that catalyzes the formation of N6-γ-glutamyllysyl bonds in fibrin. Congenital deficiency of factor XIII has been described in approximately 65 separate families. It is apparent that this is a hereditary trait, but the mode of inheritance is unclear. In this study two unrelated Finnish families with factor XIII deficiency were investigated in order to determine the pattern of inheritance. One family had only affected male members, and in the other both males and females were affected. Plasma factor XIII levels were measured by the fluorescent amine incorporation assay, which has been found to be sensitive to the detection of heterozygotes. In both families it was found that the parents of the affected children were indeed heterozygous for factor XIII, and as far as could be determined in the pedigrees each heterozygote had at least one heterozygous parent. This study indicates that in these two families the mechanism of inheritance of factor XIII deficiency is autosomal recessive. Comparison of these families with others reported in the literature suggests that an autosomal recessive pattern is the general mode of inheritance in factor XIII deficiency.