A 70-yr-old woman presented with a severe haemorrhagic diathesis due to an acquired factor X deficiency. A plasma infusion study showed that exogenous factor X was eliminated very effectively from the patient's circulation. A bone marrow biopsy was consistent with plasma cell dyscrasia. Neither an abdominal fat biopsy nor the bone marrow biopsy confirmed an amyloidosis, although clinically no other diagnosis seemed possible. Treatment with intermittent chemotherapy, consisting of vincristine, cytoxan and prednisone, yielded definite clinical and laboratory improvement.