Because nonneoplastic and reactive conditions of the central nervous system (CNS) may often mimic tumor? in their clinical and radiologic presentation, their distinction by biopsy sample may prove a challenge. Acute inflaminatory demyelinating disease, especially multiple sclerosis (MS), is of importance in this regard, but a variety of infections and other lesions come into consideration. We describe a practical Approach to the diagnosis of tumorlike lesions, one aimed at The avoidance of pathologist error. The use. of neuroimaging studies in differential diagnosis is emphasized throughout.