The long-term course of intracranial pressure (ICP) was studied in 53 patients from a group of 90 patients with severe head injury treated over a 3-year period. In 49 of these, ICP was significantly elevated during the observation period. The maximum in ICP was usually observed 24 to 96 hours posttrauma. A subgroup of patients developed a second rise of ICP. Such a course was observed in 15 (31%) of the 49 patients with intracranial hypertension. In these cases, ICP increased initially to 20 to 30 mm Hg but could be controlled. Thereafter, ICP was decreased again for at least 12 hours. The secondary ICP rise occurred 3 to 10 days after trauma. In six patients, intracranial hypertension became uncontrollable and eventually caused brain death. The outcome of patients with a secondary rise of ICP was worse when compared with that of patients without this complication. A cause of the secondary ICP rise could only be identified in some cases. Delayed traumatic intracerebral hemorrhage, traumatic vasospasm, hypoxia, and hyponatremia were diagnosed in seven cases. In seven other patients, the secondary ICP rise coincided with a pronounced leukocytosis, which was not associated with apparent infections. Because the occurrence and degree of a secondary rise of ICP after severe head injury are important factors affecting outcome, monitoring of ICP after severe head injury should be prolonged.