PEEP (positive end-expiratory pressure) was required in 12 head-injured patients in whom intracranial pressure (ICP) monitoring had been previously established. In six, ICP increased by 10 mm Hg or more as 4–8 cm H2O of PEEP were administered. In 10 patients the mean arterial pressure decreased during PEEP. Before PEEP, the mean cerebral perfusion pressure (CPP = BP-ICP) was above 50 mm Hg in all patients. The CPP was less than 50 mm Hg in six patients given PEEP. Neurological deterioration occurred in two patients during PEEP therapy. In head-injured patients, optimal titration of PEEP therapy should include ICP measurement and/or continuous evaluation of neurologic status.