Neoplastic invasion of the brain parenchyma results in a disruption of the ultrastructure of the blood vessel walls such that serum proteins extravasate into the surrounding tissue, resulting in cerebral edema. The structural changes involved are not well understood, since the pores through which serum constituents pass (permeability routes) in normal barrier vessels and in tumor vessels where the barrier is compromised, have not been extensively explored. In this study we investigate the ultrastructure of human brain microvessels in biopsied samples of control brain tissue and five glioblastoma multiforme tumors. Electron micrographs of a total of 78 vessels were analysed with computer assisted morphometry for ultrastructural evidence of permeability routes. Fenestrations in the endothelium were not seen. Pinocytotic vesicle number and arrangement did not differ significantly from that seen in control brain vessels. Interendothelial junctions with enlarged distensions (which may represent sections through transendothelial channels) were seen in some vessels from most tumors but not in control barrier vessels. In addition, large gaps in the endothelial layer were seen in less than two percent of tumor vessels. In conclusion, glioblastoma multiforme vessels in this study show subtle alterations in vessel morphology from that seen in controls. We suggest that the high vascular permeability and resultant brain edema seen in glioblastoma multiforme tumors is likely due to the presence of channels through interendothelial junctions, and rare but large breaks in the endothelial wall.