Eight peripheral and 2 central oral giant cell granulomas have been studied in the electron microscope in order to examine the structure and nature of the vessels. The vascular wall was composed of a continuous layer of endothelial cells resting on a basal lamina, which was frequently multilayered. The lamina also surrounded scattered subjacent pericytes. The endothelial cells overlapped slightly so as to form narrow intercellular spaces. Based on these observations the vessels are supposed to be postcapillary venules. Dark cells exhibiting degenerative features were encountered in the endothelial lining of most vessels. Small gaps and larger defects in the vascular lining allowed direct continuity between the lumen and the perivascular tissue. It is suggested that the extreme extravasation of red blood cells frequently observed in oral giant cell granulomas may be mediated through this incomplete lining.