A selective acquired Factor X deficiency is an unusual occurrence. Six cases of an acquired Factor X deficiency in association with amyloidosis have been reported. This paper describes two additional cases, suggesting that this relationship may be more than coincidental. The mechanism by which amyloid may affect Factor X levels remains unknown, but suggestions include consumption, inactivation or decreased synthesis of Factor X. Factor II, VII, IX, and X concentrate transiently increased the Factor X level to normal in one of the patients. In an adult patient who has an isolated Factor X deficiency, amyloidosis should be actively sought.