Ten patients with peripheral polyneuropathy associated with plasma cell neoplasias are reported. Progressive sensorimotor polyneuropathy was the presenting complaint in all patients. CSF protein concentration was elevated in most patients. The electrophysiological and pathological changes were consistent with a primary segmentary demyelinating disease. All patients were male and younger than the average patient with myeloma. None presented with high tumour mass or overt multiple myeloma. Six were affected with single or multiple plasmacytomas with osteolytic lesions. Unusual haematological features such as polycythaemia, thrombocytosis and lymphadenopathy were often combined with the polyneuropathy. Skin hyperpigmentation, gynaecomastia and diabetes mellitus were noted in some patients. Complete recovery of the polyneuropathy was observed in some patients after either cyclophosphamide and corticosteroid therapy or radiotherapy of localized plasmacytoma, suggesting a direct relationship between the plasmacytic proliferation and the neuropathy. The nature of the postulated factor produced by the plasma cells and responsible for nerve injury is discussed and the importance of a careful search for plasma cell proliferation in men with obscure polyneuropathies is outlined.