The authors used light and electron microscope to examine the capsules of chronic subdural hematoma in 33 cases. In cases with neurological deficits, capillary endothelial cells in the capsule had many cytoplasmic protrusions and fenestrations, suggesting high permeability of the capillary wall. Endothelial degeneration was also observed in these cases. These morphological changes were reversed by osmotherapy. Formation of collagen fibrils from fibroblasts in the hematoma capsule was frequently observed in the cases treated by osmotherapy.