A 40-year-old man was admitted to our hospital because of paresthesia and weakness of the limbs. At the age of 38, he was diagnosed as having an idiopathic thrombocytopenic purpura (ITP) which have been refractory to oral administration of prednisolone and splenectomy. Platelet-associated IgG was elevated markedly at that time. It was, however, only mildly elevated on this admission. He showed polyneuritis, generalized pigmentation, hirsutism, and marked edema on the legs. The bone X-ray disclosed a lytic lesion in the left iliac bone, which was confirmed as a plasmacytoma by bone biopsy. Axonal degeneration with marked loss of myelinated figure was seen on sural nerve biopsy. Serum immunoelectrophoresis revealed his monoclonal IgG was lambda type. Then, he was diagnosed as having a Crow-Fukase syndrome associated with ITP. Plasma exchange, pulse therapy, and irradiation to plasmacytoma resulted in a slight improvement of the polyneuritis and the skin symptoms, and a disappearance of edema. However, ITP has not responded to these therapies. Although the same autoimmune mechanism is suggested in these conditions, we could not clarify how this monoclonal IgG produce both polyneuritis and ITP.