Irradiation of gliomas inevitably involves irradiation of surrounding normal appearing brain. We analysed longitudinal, quantitative MR data of 24 glioma patients before and at 3, 6, 9 and 12 months after radiotherapy and found significant reductions in mean-, axial- and radial diffusivity as well as in T2* in normal appearing white matter. These changes are greater the higher the received dose and progress over time. The diffusion reductions point towards axonal swelling. T2* reductions indicate either increased tissue heterogeneity, e.g. due to microglial activation or changes in tissue oxygenation, e.g. due to vascular alterations.