AL amyloidosis is related to myeloma in two ways. Firstly, as in myeloma, abnormal plasma cells in the bone marrow are source of the pathology. Plasma cells are part of the immune system that, under normal conditions, make antibodies to fight infections. Plasma cells make a certain type of proteins called immunoglobulins, which act as antibodies. They are composed of four protein chains, two light  chains (called kappa and lambda) and two heavy chains. In AL amyloidosis patients, plasma cells produce an abnormal immunoglobulin light chain which is deposited in tissues and organs as amyloid, causing damage and affecting organ function. Both diseases are related also in the sense that amyloidosis treatment uses the same procedures and drugs as myeloma treatment due to the similarity in their pathophysiology and the need to abolish the malignant plasma cell clone.

Approximately 12–15% of myeloma patients will develop amyloidosis, some with subclinical amyloid deposits, which means the amyloidosis is not fully manifested. In both cases this disease is called multiple myeloma associated amyloidosis and it is treated in the same way as myeloma.