The aetiology, mechanisms of haemorrhage and results of treatment of a series of 63 consecutive cases of acute subdural haematoma are presented. Forty-six cases were due to trauma (46% mortality) and 17 cases were non-traumatic (41% mortality). Only 15 patients overall (24%) made a good recovery. The various mechanisms of venous and arterial bleeding leading to subdural haematoma are reviewed and the importance of brain swelling as a determinant of fatal outcome is stressed.