The relationship between plaque pathology and disease duration was examined in 15 patients with multiple sclerosis who died early in the course of their illness. Myelin-stained sections revealed that most plaques examined in patients who died during the first month of their illness showed evidence of ongoing myelin destruction accompanied by a loss of oligodendrocytes. Plaques containing large numbers of oligodendrocytes were not observed in these patients, but were relatively common in patients who died more than 1 month after clinical onset. Remyelination affecting more than 10% of the plaque area was observed in 3 of 82 plaques in 5 patients who died within 10 weeks of clinical onset, in 38 of 105 plaques in 5 patients who died 3 to 10 months after clinical onset, and in 19 of 92 plaques in 5 patients who died 18 months or longer after clinical onset. The study provides new evidence that both oligodendrocytes and myelin are destroyed in new lesions, that this activity ceases completely in many lesions within a few weeks, and that remyelination frequently ensues following repopulation of the plaque by oligodendrocytes. The findings suggest that new lesions normally remyelinate unless interrupted by recurrent activity and that remyelinated shadow plaques are the outcome of a single previous episode of focal demyelination.