1. Raised intracranial pressure has been noted in severe forms of acute mountain sickness and high-altitude cerebral oedema, but the role of intracranial pressure in the pathogenesis of mild to moderate acute mountain sickness is unknown.
2. Serial measurements of intracranial pressure were made indirectly by assessing changes in tympanic membrane displacement in 24 healthy subjects on rapid ascent to 5200 m.
3. Acute hypoxia at 3440 m was associated with a rise in intracranial pressure, but no difference was found in pressure changes at 4120 or 5200 m in subjects with or without symptoms of acute mountain sickness.
4. Raised intracranial pressure, though temporarily associated with acute hypoxia, is not a feature of acute mountain sickness with mild or moderate symptoms.