From January 1, 1990, to April 30, 1994, 412 patients were admitted to our intensive care unit in coma after head injuries. Our study group consisted of 37 patients who were retrospectively identified as harboring lesions or developing new lesions within a 12-hour period from the time of admission. We defined the evolution of a lesion as an increase or decrease in the size of an already present hematoma or as the appearance of a totally new lesion. There were 25 male and 12 female patients (mean age, 34.9 yr), and the cause of trauma was road traffic accidents in 32 patients. Nine patients presented with shock, and six had evidence of abnormal coagulation at admission. Patients were divided into two different groups. In Group 1, 15 patients harbored lesions that evolved toward reabsorption. In Group 2, 22 patients harbored hematomas that evolved toward lesions requiring surgical removal. Fifteen of these patients had initial diagnoses of diffuse injury that evolved in this manner, whereas the remaining seven patients had already been operated upon and had developed second, noncontiguous, surgical lesions. Patients with lesions that required surgical evacuation had their computed tomographic (CT) scans obtained earlier and had a higher incidence of clinical deterioration. There was a significant difference in the evolution of the different lesions (P < 0.001), with subdural hematomas being more prone to reabsorption and intracerebral and extradural hematomas being more likely to increase in size or to appear as new lesions. Second CT scans were obtained because of clinical deterioration in 10 patients and because of increase in intracranial pressure in 5 patients. Scheduled CT scans were obtained in 13 patients, whereas in the remaining 9 patients, the diagnosis emerged from a combination of scheduled CT scans and intracranial pressure monitoring. There was a trend toward a poorer result among the patients with clinical deterioration, which, however, was not significant. A significant proportion of post-traumatic patients, particularly those who are unconscious, harbor early evolving intracranial lesions. When the first CT scan is performed within 3 hours after injury, a CT scan should be repeated within 12 hours.