The authors retrospectively reviewed the cases of 40 patients with biopsy- or autopsy-proved inflammatory demyelination consistent with multiple sclerosis (MS); each patient underwent magnetic resonance (MR) and/or computed tomographic (CT) imaging. In review of the 32 MR and 30 CT examinations, three predominant radiologic patterns were apparent: (a) relatively small homogeneous lesions with no or minimal diffuse enhancement; (b) hypoattenuating lesions on CT scans and hypointense lesions on short-TR MR images, with a contrast material-enhanced, isoattenuating or isointense ring; and (c) lesions that were more infiltrative and ill defined, with mixed attenuation and signal intensity and scattered enhancement. A common radiologic feature was lack of mass effect or edema in white matter surrounding even large lesions. In all but four patients the lesions were classified as active by using both histologic and clinical criteria. Histologically active lesions had various radiologic appearances; however, in all contrast-enhanced studies they demonstrated some form of enhancement. Inactive lesions were homogeneously hypoattenuating on CT scans, hypointense on short-TR MR images, and hyperintense on unenhanced, long-TR MR images.