The exact cause of AL amyloidosis is not known. The condition appears to occur in similar frequencies among people of various races. It is thought to occur more commonly in men than in women. AL amyloidosis is a disease of adulthood. The mean age of people at diagnosis of AL amyloidosis is 65 years. The condition results from an abnormality of plasma cells in the bone marrow. It is closely related to multiple myeloma.