Cerebrospinal fluid (CSF) dynamics were correlated to the changes in ventricular size during the first 3 months posttrauma in patients with severe head injury (Glasgow Coma Scale score < or = 8, 75 patients) to distinguish between atrophy and hydrocephalus as the two possible causes of posttraumatic ventriculomegaly. Using the bolus injection technique, the baseline intracranial pressure (ICP), pressure volume index, and resistance for CSF absorption (R0) provided a three-dimensional profile of CSF dynamics that was correlated with ventricular size and Glasgow Outcome Scale (GOS) score at 3, 6, and 12 months posttrauma. Patients were separated into five different groups based on changes in ventricular size, presence of atrophy, and CSF dynamics. Group 1 (normal group, 41.3%) demonstrated normal ventricular size and normal ICP. Group 2 (benign intracranial hypertension group, 14.7%) showed normal ventricular size and elevated ICP. Group 3 (atrophy group, 24%) displayed ventriculomegaly, normal ICP, and normal R0. Group 4 (normal-pressure hydrocephalus group, 9.3%) had ventriculomegaly, normal ICP, and high R0. Group 5 (high-pressure hydrocephalus group, 10.7%) showed ventriculomegaly and elevated ICP with or without high R0. The GOS score in the nonhydrocephalic groups (Groups 1, 2, and 3) was better than in the hydrocephalic groups (Groups 4 and 5). It is concluded from these results that 44% of head injury survivors may develop posttraumatic ventriculomegaly. Posttraumatic hydrocephalus, as identified by abnormal CSF dynamics, was diagnosed in 20% of survivors and their outcome was significantly worse. This study demonstrates the importance of using CSF dynamics as an aid in diagnosis of posttraumatic hydrocephalus and identifying those patients who may benefit from shunt placement.