Congenital Factor X deficiency is commonly classified as type I, in which there is a concomitant decrease of activity and antigen (CRM negative), and in type II, in which activity is low but antigen is normal or near normal (CRM positive). During the past decades it was shown that type II was by itself very heterogeneous. It was shown in fact that some forms showed a defect in all three assay systems (extrinsic, intrinsic, and RVV dependent), whereas others showed a defect only in two of the three systems. Molecular biology analysis, whenever available, has failed so far to supply clear explanations for these discrepancies. The purpose of the present article was an attempt to correlate the clotting activities seen in these two defects with other clotting, chromogenic, immunological assays, and molecular biology results. There are in the literature 10 families that show a predominant defect in the extrinsic system, and four families that show a predominant defect in the intrinsic system. All patients showed a normal, near normal, or reduced level of antigen that is always definitively higher than the clotting counterpart. Molecular biology studies revealed mutations in different exons, namely 2, 4, 5, 6, and 8. These mutations in different exons do not allow any clear genotype-phenotype conclusions, but indicate that mutations in different exons may give rise to the same phenotype. The study underlines the importance of a multipronged evaluation of all cases with Factor X deficiency. In fact only by this approach can an acceptable classification of the defect be reached. Am. J. Hematol., 2008. © 2008 Wiley-Liss, Inc.