A group of 29 patients with congenital factor XII (FXII) deficiency belonging to nine distinct families have been investigated. All were cases of true deficiency in the sense that there was no discrepancy between FXII activity and FXII antigen. From a clotting point of view, 11 patients appeared homozygous, as both FXII activity and antigen were very low (C in the promoter region confirming the conclusions reported by coagulation tests. On the contrary, the remaining patients with a homozygote-like phenotype were instead found to be compound heterozygous for two distinct mutations. Three of these mutations were new mutations, namely the combination of -8G to C with 501Q to T (exon 13), 547P to L (exon 14) and -13C to T in the promoter, respectively. The remaining mutations seen were not new. It is interesting that all compound heterozygotes showed a clotting and immunological pattern similar to that shown by homozygotes, namely very low FXII activity and antigen. The new mutations were not present in the group of 98 normal persons of both sexes with the same geographical background. The wide diffusion of the -8G>C mutation in this group of patients coming from a limited geographical area suggests a founder effect. The significance and importance of genetic analysis in addition to clotting and immunological studies in FXII deficiency is emphasized.