The vascular response to irradiation was studied in peritumoral cortex and tumor areas in 9 autopsied cases of glioblastoma multiforme and in 10 biopsies of reoperated cases. The study was carried out by histological, immunohistochemical and electron microscope procedures. Hyaline thickening of the vessel walls with deformation and impoverishment of endothelial nuclei are the most striking features. Formation of large PAS-positive areas from the confluence of thickened vessel walls and development of a fibroblastic proliferation follow. Fibroblastic areas may entrap tumor glial cells. In PAS-positive areas a special population resembles myofibroblasts of tissue scars. Electron microscope reveals the production of collagen fibrils in and outside vessel walls. The presence of collagen III and IV is in line with the present interpretation of the mesodermic component.