We report the first antemortem diagnosis of a lesion showing Balo's concentric sclerosis. A patient with a progressive left hemiparesis had a ring-enhancing, low-density right frontal white matter lesion. On myelin stains of a needle biopsy, alternating demyelinated and myelinated zones in the white matter were diagnostic of concentric sclerosis. The patient improved with prednisone therapy, but relapsed temporarily 15 months later. CT and MRI showed additional lesions, but no features unique to this process. He remains alive and employed 3 years after diagnosis.