An ultrastructural study was performed to define the mechanisms of proliferation of macrocapillaries and capillaries in the capsule of chronic subdural hematoma. Mitotic activities of endothelial cells and vascular sprouts may contribute to the vascular proliferation. Platelets may also play an important role. Endothelial cells of proliferating macrocapillaries were highly attenuated except for the nuclear portion and showed variable electron densities. The adjacent endothelial cells had less intimate cellular junctions than the proliferating capillaries. The proliferating endothelial cell sometimes contacted the opposing endothelial cell across the lumen, separating the latter into two compartments. The pericytes varied remarkably in size, shape, and electron density. Proliferating capillaries had a narrowed vascular lumen and were characterized by cuboidal or thickened endothelial cells with enlarged nuclei, numerous pinocytotic vesicles, and cytoplasmic protrusions into the vascular lumen. They occasionally had intimate cellular junctions with interdigitations and slender pericytes of high electron density. Proliferating vessels occasionally had some clear endothelial cells that contained filamentous material and a few dense bodies. These cells might represent degenerated or swollen endothelial cells. However, they showed no evidence of increased permeability. Postdegranulation platelets were sometimes trapped in the endothelial linings. These data indicate that the most important factor for the development of chronic subdural hematoma exists in the vessels of the capsules, which have a marked proliferation potential and a fragile nature. The possibility of transition from proliferating capillaries to macrocapillaries is discussed.