Hematomas in head injuries as a general rule reach their definite size within minutes after the trauma, the bleeding being effectively checked by an interaction of an increased intracranial pressure and the natural hemostatic processes. In epidural hemorrhage the development of arteriovenous shunting in the epidural space may result in continuing bleeding. In special circumstances vascular injury may produce delayed hemorrhage related to increased transmural pressure in the vascular bed and the development of a hyperfibrinolysis syndrome. The clinical effect of a hematoma is quantitatively related to its volume, but modified to a considerable degree in the particular case by the size of the extraaxial space and the arterial blood pressure. Some implications for treatment are commented upon.