A 54-year-old woman with a four-month history of progressive neurological illness was found at postmortem examination to have lesions of Balo's concentric sclerosis. Balo lesions were found in several areas scattered widely throughout the central nervous system, including the spinal cord, a previously unreported location, and were studied by histological and ultrastructural methods. Balo lesions consisted of bands of intact myelin alternating with zones of demyelination. These lesions were centered on a perivascular cuff of inflammatory cells. The center of the lesion was the oldest area with the concentric rings of demyelination decreasing in age with increasing distance from the center. The bands of intact myelin comprised mainly remyelinated fibers, were similar to those seen at the edges of chronic active multiple sclerosis plaques, and may have represented the repaired margins of preceding episodes. The occurrence of small foci of acute demyelination centered on perivascular cuffs and other changes typical of both acute and chronic active multiple sclerosis may indicate that the lesion of Balo's concentric sclerosis represents an intermediate stage in the development of an established multiple sclerosis lesion.