A patient with high-grade multi-focal glioma deteriorated rapidly despite high dose corticosteroid therapy (dexamethasone: 16 mg/day). MRI was used to measure diffusion tensor parameters and longitudinal relaxation time (T1) values of peritumoural oedematous brain before and after commencing steroid treatment. Forty-eight hours after steroid treatment there was no evidence of brain oedema reduction. Specifically, regions of oedematous brain showed a significant increase in mean diffusivity (<D>) with a significant decrease in diffusion anisotropy (p < 0.05), but without any change in T1 values. These quantitative MRI data were mirrored by the rapid deterioration seen when assessing the patient clinically. This case shows that quantitative MRI can not only measure steroid treatment response but also failure in patients with malignant gliomas.