&NA;The effects of positive end‐expiratory pressure (PEEP) on central venous and intracranial (ICP) pressures were evaluated in 10 patients with posterior fossa tumors, in both supine and sitting positions. With patients in the supine position, intrathoracic PEEP‐dependent venous hypertension was clearly transmitted to the intracranial compartment but without intracranial hypertension. On the contrary, with patients in the sitting position PEEP had no influence in almost half of our patients. In patients with radiological or clinical signs of increased ICP, however, the combination of head flexion and rotation with institution of PEEP caused a dangerous increase in ICP, even when the patients were in the sitting position. The need for early withdrawal of cerebrospinal fluid in these patients is stressed. (Neurosurgery24:873‐877, 1989)