Previous investigations have reported an 11 to 71% incidence of pulmonary edema following CNS injury, based on post-mortem examinations. No premortem assessment to date has been made of the frequency and severity of in vivo pulmonary fluid accumulation following acute head injury. The thermal green dye technique was used to objectively determine extravascular lung water (EVLW) in 18 comatose patients with severe acute intracranial injuries resulting from trauma or spontaneous subarachnoid hemorrhage. Patients with aspiration, lung contusion, or pneumonia were excluded from the study. Control values for EVLW were obtained on admission from a group of 13 traumatized patients without head injury or evidence of pulmonary disease or injury. Intracranial injury patients who had EVLW values greater than two standard deviations above the control group mean were considered to have pulmonary edema. The incidence of pulmonary edema in this series was 50% (9/18). Extravascular lung water was determined post mortem in five patients using the gravimetric method of Pearce. The results suggest that pulmonary edema is a distinct clinical event occurring frequently after acute intracranial injury. Edema appears to be mediated by increased pulmonary microvascular permeability, and is not primarily dependent on postinjury changes in intracranial pressure or pulmonary vascular pressures.