We examined the specimens of tumors from three patients with orbital cavernous hemangioma by light and electron microscopy. The tumors mostly consisted of many vessels of various sizes. A part of the tumor tissues showed irregular vascular inner spaces like sinusoids. Apart from the sinusoid-like vessels, we divided the other vessels into 10 groups at every 100 microns-interval based on the difference of the inner diameter and measured the maximum and minimum width of the media. The average maximum width increased until the inner diameter reached 300-400 microns, while the average minimum width increased until it reached 200-300 microns. In groups with greater inner diameter, the average maximum and average minimum widths were approximately constant. There were three kinds of vessels: vessels of cuboid endothelia, of flat endothelia, and of both cuboid and flat endothelia. The width of media of the vessels having cuboid and flat endothelia was greater than that of the other types of vessels. The lamina densa of endothelia was broken or stratified. Myofibroblast-like cells proliferated among the vessels and could not be distinguished from tumor cells of hemangiopericytoma. These results indicate that smooth muscle cells proliferate by an unknown mechanism and proliferation is associated with changes of endothelia in cavernous hemangioma, and strongly suggest that this tumor is hamartoma.