Axial MRI and MR spectroscopy data acquired at 3 Tesla in a 30 year old woman with tumefactive MS. Techniques used were fluid-attenuated inversion recovery (FLAIR; A-C, H-J, P, Q), diffusion weighted imaging (D W I; D) and apparent diffusion coefficient (ADC; E) maps, T1 before (F, K) and after administration of gadolinium contrast (G, L, O), respectively. At presentation (A-G), FLAIR showed a large area of high signal in the left occipito-temporal region including the angular gyrus (arrow in A) and extending to the genu of the corpus callosum (A-C), while DWI and ADC maps showed partially restricted diffusion (D, E). Significant enhancement occurs after injection of gadolinium (G). Two weeks after presentation (H-L) FLAIR (H-J) showed increase in size of the index lesion, and three additional lesions, two in the frontal lobes (H, I) and one in the left occipital lobe (data not shown). All four lesions enhanced after application of gadolinium (only index lesion shown in L). Following brain biopsy, and four weeks after the first natalizumab infusion (M-O), there is significant reduction in size of the index lesion with evidence of post-surgical cavity and minimal residual gadolinium enhancement (O) Fourteen months after first presentation, and after 13 courses of natalizumab, no evidence of disease activity was detected (P, Q). Long echo MR spectroscopy of the tumefactive lesion at presentation revealed significantly reduced NAA/Cr ratio, increased Cho/Cr ratio and an inverted lactate doublet curve (arrow) (R).