THsi study was designed to define the effect of positive end expiratory pressure (PEEP) ventilation on intracranial pressure (ICP). In 25 patients with severe head trauma with and without associated pulmonary injury the following parameters were simultaneously monitored under mechanical ventilation with and without PEEP:ICP, arterial blood pressure, central venous pressure, arterial blood gases, and cardiac rate. In addition, the volume-pressure response (VPR) was evaluted in each patient to assess cerebral elastance. The results indicate a significant increase in ICP with the application of PEEP only in the 12 patients who manifested increased cerebral elastance by VPR. Half of this latter group manifested impairment of cerebral perfusion pressure to levels less than 60 mm Hg. Return to baseline CIP levels was observed with termination of PEEP. No significantly consistent changes in other parameters were noted.