Tumefactive demyelinating lesions (TDL), defined as inflammatory demyelinating lesions, may develop either during treatment for multiple sclerosis and related disorders or as the first demyelinating episode without any past medical history suggesting demyelination. If the latter, it would be so delicate to diagnose as demyelination. Especially in such situations, biopsy is often necessary in addition to neuroimaging for distinction TDL with tumorous or infectious diseases. In this article, we will review about concept, epidemiology, diagnosis, and treatment of TDL.