Follow-up results for 13 cases of acute subdural haematoma, which were identified by CT scan within two days of head injury and treated conservatively, were classified into two groups: In the first group of seven cases the acute subdural haematoma disappeared spontaneously within two weeks; in the second group the other six cases revealed clinical signs and CT scan findings which were identical to chronic subdural haematoma and underwent surgery within three weeks after the head injury. This group showed low ICP signs, such as the chronic stage of cerebral thrombosis, atrophic brain or subdural fluid collection with acute subdural haematoma at the initial CT scan, and most of these cases had only minor head injury. Common features of the operative findings in this second group, in which chronic subdural haematoma developed, included the identification of an external haematoma capsule based on the fibrin layer and granulation tissue beneath the dura. Also, fluid type haematoma was present under these structures, but no inner membrane of the haematoma could be identified other than the non-transparent white arachnoid membrane. Our results indicate the importance of a low ICP in determining whether or not acute subdural haematoma progresses to chronic subdural haematoma.