We studied the electron microscopic features of the outer membrane of chronic subdural hematoma to explore the mechanism of growth of chronic subdural hematoma. Ultrastructurally, the outer membrane consisted of bundles of collagen fibrils and cellular elements such as fibroblasts, mast cells, migrating erythrocytes, platelets and eosinophils. A large number of proliferating macrocapillaries coursed among them. Such general characteristics of the endothelial cells with proliferating macrocapillaries as gap junctions and thinness or absence of the basement membrane suggested that they be very fragile and susceptible to bleeding. The number and extent of endothelial gap junctions indicated that they could account for most of the leakage not only into the tissue of the outer membrane but also into the hematoma cavity. The outer membranes had a prominent infiltration of eosinophils. Plasminogen secreted by the eosinophils inhibits the formation of platelet thrombus within the lumen and also dissolves fibrinoid substance, which reinforces fragile endothelial walls or edematous interstitium. These features possibly contribute to the recurring hemorrhage from the vessels in the outer membrane and the resultant enlargement of the hematoma.