Acquired factor X deficiency has been described in association with primary amyloidosis in a small number of patients. Although readily corrected in vitro by mixing patient plasma with normal plasma, the deficiency is not easily corrected in vivo because transfused factor X is rapidly cleared from the circulation. It has been suggested that the mechanism underlying this phenomenon may be the selective binding of factor X to amyloid and its consequent removal from the circulation. Two cases of primary amyloidosis and coexistent acquired factor X deficiency are reported. Immunostains for factor X and immunoglobulin light chains showed the coagulation factor in the region of the amyloid deposits. The findings directly support the hypothesis that factor X deficiency in primary amyloidosis is caused by clearance of factor X by binding to amyloid fibrils.