RESUMEN
Factor X deficiency constitutes one of the most uncommon congenital bleeding disorders. Here we report three families with Factor X deficiency, one of them with an associated deficit of Factor XII. Family I presented Red variant deficiency (low functional and antigenic activity, the latter in higher levels than the former). In Family II functional activity was low but antigenic one was normal (Prower defect). Besides, an heterozygous deficiency of factor XII was diagnosed. Although genetic analysis supports the hypothesis of combined deficiency, the study was possible in only two generations of the propositus, so a multiple familial deficiency could not be discarded. Finally, Family III suffered from a "classic" or Mr. Stuart deficiency (low levels in functional and immunological assays). Besides, crossed immunoelectrophoresis showed a grossly pathological pattern.