Factor X deficiency is a severe rare hemorrhagic condition inherited as an autosomal recessive trait. It is one of the most severe recessive inherited coagulation disorders. We analyzed the clinical manifestations, laboratory phenotype and genotype in 10 patients with severe Factor X deficiency and in their heterozygous relatives. The most frequent bleeding episodes were hematomas (70%) and gum bleeding (60%). Fifty percent of the homozygous patients required blood transfusion and one-third of heterozygotes required treatment after surgery or delivery. The genetic characterization revealed six different missense mutations, two of which were novel: p.Glu69Lys and p.Asp103His. Haplotype analysis, performed with intra- and extra- FX gene polymorphic markers in Indian, Iranian and Italian patients with the same mutations failed to establish identity by descent, despite the same Caucasian origin. In conclusion, factor X deficiency was confirmed to be one of the most serious among rare bleeding disorders and genetically heterogeneous in different populations.