The effects of Paco2changes on intracranial pressure (ICP) often remain even after prolonged hyperventilation. Two representative cases are described where Paco2rises were directly related to serious rises in ICP after 4, 6, and 14 days of hyperventilation. This is common in head injuries and suggests that where hyperventilation is used to reduce ICP in the presence of brain swelling, withdrawal of the technique should be cautious.