A new case of extensive subacute polyradiculoneuritis associated with a costal plasmocytoma secreting IgG lambda is reported. Radiotherapy of the rib, resulted in remarkable neurological improvement and haematological cure, which have now persisted for more than 5 years, though treatment was discontinued 3 years ago. By its distinctive haematological features, this case illustrates the lack of clear-cut distinction between "solitary" plasmocytoma and multiple myeloma. It also emphasizes the need for systematic testing for plasma cell proliferation when confronted with a subacute or chronic neuropathy of obscure origin.