Difficulties may arise in the diagnosis of patients with clinical features suggestive of plasma cell dyscrasia-related amyloidosis (amyloidosis L), but without evidence of a paraprotein. We have employed gene rearrangement methodology to demonstrate the clonality of bone marrow cells not only in a patient with myeloma-associated systemic amyloidosis, but also in a patient with "primary" systemic amyloidosis without overt myeloma or a detectable paraprotein. Furthermore, we have shown the clonality of the amyloid-producing plasma cells within a skin nodule of a patient with primary localized cutaneous amyloidosis; by contrast, clonal rearrangement was not detected in bone marrow cells from this patient. This finding provides definitive proof that organ-limited nodular primary localized cutaneous amyloid deposits arise in relation to cutaneous plasmacytomas. Gene rearrangement studies may enable early diagnosis and initiation of treatment in patients with systemic amyloidosis L, as well as their differentiation from patients with organ-limited nodular cutaneous amyloidosis, who do not require aggressive therapy.