A 63-year-old man was admitted to our hospital with tremor and somnolence, followed soon by coma. Anemia and retinal bleeding were observed. The blood smear exhibited rouleaux formation and leukoerythroblastosis. A bone marrow aspiration resulted in dry tap. The biopsy specimens revealed remarkable infiltration of myeloma cells with fibrosis. The M-component of IgG-lambda type and hyper-ammonemia were detected in the serum. Liver and renal functions, however, were within normal range. His consciousness recovered after plasmapheresis. Two courses of VMCP (vincristine, melphalan, carboquone and prednisolone) did not affect the paraproteinemia. Five courses of VAD (vincristine, adriamycin and dexamethasone) could lower the level of IgG. He died of pneumonia. The plasma of some patients with multiple myeloma may contain unidentified factors which increase the plasma ammonia.