Twelve cases of severe altitude illness are reported in which the neurological signs and symptoms dominated the clinical picture. Pulmonary oedema, retinal haemorrhage, thrombophlebitis and pulmonary embolism, bronchopneumonia, and coronary-artery disease were also present in several of the patients but the primary problem seems to have been cerebral oedema. Other published cases support this impression. Patients who were returned to low altitude early in the disease fared well; two patients died, and in both cases evacuation had been delayed. The most effective prevention lies in slow ascent, though in one case reported here the rate of climb was well within the recommended limit. Recommended management is rapid descent to low altitude at earliest indication of cerebral or pulmonary oedema, intravenous dexamethasone or betamethasone in large doses, hydration, diuresis (frusemide has been most used), and perhaps other intravenous therapy with hyperosmolar materials such as mannitol, urea, 50% saline, or 50% sucrose. Prognosis is good if descent and treatment are started early, but permanent damage may be anticipated if the patient is unconscious for any prolonged period before descent.