Traumatic subdural hematoma is a definite clinical entity that has been given considerable attention in the medical literature since Virchow's classic description1of "hematomas of the dura mater" in 1857. The lesion consists of an encysted collection of blood, situated between the dura and the arachnoid membranes, usually over the convexity of the cerebral hemisphere. It is my aim to sketch briefly the clinical and pathologic aspects of this interesting lesion and to dwell particularly on the reason for its delayed clinical manifestations. The majority of published reports of cases of traumatic subdural hematoma are similar in one respect, that is, in the occurrence of a latent interval between the reception of trauma and the onset of pressure symptoms. The cranial trauma that was responsible may have been severe or so trivial as to be readily forgotten. In many instances a history of trauma that had been denied previously