Out of a group of 97 patients with intracranial haematomas, 12 were found to have a traumatic subdural hygroma. In four patients a small fluid effusion was demonstrated by CT scan just after the head injury. In the majority of cases the CT scan demonstrated a significant increase of volume and pressure of the subdural effusion during the second to third week. Half the patients simultaneously developed a moderate ventricular dilatation. The development of a subdural effusion was hard to recognize from the clinical course. Nevertheless the patients mostly had a good post-operative recovery. The hygromas disappeared by subdural peritoneal shunting. In two patients a ventriculoatrial shunt was necessary on account of the hydrocephalus. Based upon clinical and CT scan characteristics a multifactorial hypothetical model of the development of the traumatic subdural effusion is proposed.