A 30-year-old man who had been given a diagnosis of IgG-kappa multiple myeloma by another hospital and treated with melphalan, prednisone, and cyclophosphamide 6 months earlier, was admitted to our hospitaly in July 1994 because of progressively impaired hearing in both ears, vertigo, and worsening fatigue. Peripheral blood examination showed a white blood cell count 25,000/microliter, with 77.5% atypical plasma cells. Examination at the time of hospitalization also revealed retinal hemorrhages and serum hyperviscosity. The diagnosis was plasma cell leukemia with hyperviscosity syndrome. Subsequent treatment consisted of vincristine, doxorubicine, and prednisone and repeated plasmapheresis. This resulted in a partial response and a reduction of serum viscosity but no reversal of hearing loss. One month after admission, left sixth cranial nerve plasy was demonstrated. Cranial computed tomography studies disclosed a tumoral mass in the sphenoid sinus. The patient received local radiotherapy and intensive chemotherapy, but exhibited no notable alleviation of his cranial nerve palsy. He died of septicemia and progressive disease in August 1994. This case was rare in that it involved plasma cell leukemia and bilateral neurosensory hearing loss associated with serum hyperviscosity and sixth cranial nerve plasy due to plasmacytoma within the sphenoid sinus.