Magnetic resonance imaging is a highly sensitive method for the detection of the lesions of multiple sclerosis and renders possible the study and the evolution of early lesions. Previous reports on magnetic resonance imaging following gadolinium-diethylenetriamine pentaacetic acid (Gd-DTPA) injection demonstrated that new lesions can be recognized by contrast enhancement. The pathological basis of these observations is uncertain. We have had the opportunity to study at autopsy the brain of a patient with chronic progressive multiple sclerosis who suffered acute worsening leading to death. Magnetic resonance imaging performed 10 days and 4 weeks prior to death showed new Gd-DTPA-enhanced lesions in the posterior hemispheric white matter adjacent to the lateral ventricles. Light microscopic examination of these areas demonstrated them to be fresh lesions comprising intense inflammatory activity and dense perivascular cuffs within an edematous lesion center and a striking parenchymal mononuclear cell infiltration at the margins of the lesions. Lesions that were demonstrated by increased signal on T2-weighted images, but were not enhanced following administration of Gd-DTPA, were all of the chronic type, either inactive or active. None of these showed the intense inflammatory activity of the acute lesions and most displayed fibrous astrogliosis.