A previously healthy individual began to complain of chest pain and shortness of breath at age 46 years. Severe normocytic anemia with a hemoglobin level of 7.9 g/dL and a hematocrit of 24% was found. A few weeks later, he noticed progressive numbness, weakness of both lower extremities, and poor balance. An examination in November 1997, approximately 5 months after the onset of neurologic symptoms, revealed an intact mental status, normal cranial nerve evaluation findings, and full strength. Deep tendon reflexes were grade 2+ on the Medical Research Council scale in the upper extremities and abnormally brisk in the lower extremities, with bilateral plantar flexor response. A sensory examination demonstrated markedly reduced positional sensation in the lower extremities, and it was completely absent in the toes and ankles; vibration sensation was absent in the toes, reduced in the ankles, but normal in other segments. The patient had signs of sensory ataxia without any cerebellar signs; his gait was wide based. Results of magnetic resonance imaging of the whole spine were unremarkable. Other testing, including human immunodeficiency virus and human T-lymphotropic virus 1 titers, serum levels for very long-chain fatty acids, methylmalonic acid, acid, and arylsulfatase A, heavy metal screening, VDRL test, vitamin E and B12 levels, serum protein electrophoresis, and urine immunoelectrophoresis, showed normal results. An analysis of his cerebrospinal fluid demonstrated elevated levels of protein at 82 mg/dL; the rest of the findings from cerebrospinal fluid analysis were unremarkable. Results of an electromyographic examination with nerve conduction studies were within normal limits.