During a 4 1/2-year period, seven patients with delayed onset of an extradural hematoma were seen among 80 consecutively treated cases of extradural hematoma for a frequency of 8.75%. The hematomas were insignificant or not present on initial computerized tomography (CT) scanning. Repeat CT scans within 24 hours of admission showed sizeable hemorrhages. Six hematomas were evacuated, and one was reabsorbed spontaneously. In only one patient did neurological deterioration herald the onset of the extradural hematoma, four patients remained unchanged, and two improved before diagnosis. Intracranial pressure (ICP) was monitored in five patients, four of whom showed intermittent rise in pressure despite preventive treatment. Intracranial hypotension and rapid recovery from peripheral vascular collapse seemed to be contributory factors in the delayed onset of an extradural hematoma. Awareness of this entity, a high degree of vigilance, ICP monitoring, and repeat CT scanning within 24 hours of injury are strongly recommended in these cases, especially after decompression by either surgical or medical means, recovery from shock, or whenever there is evidence of even minimal bleeding under a skull fracture on the initial CT scan.