In a laboratory at 4559 m six subjects with high altitude pulmonary oedema (HAPO) characterised by clinical signs, severe hypoxaemia, widened alveolar-arterial oxygen gradient, pulmonary hypertension, and alveolar oedema on chest radiography were treated with nifedipine. Despite continued exercise at the same altitude this treatment, without supplementary oxygen, resulted in clinical improvement, better oxygenation, reduction of alveolar arterial oxygen gradient and pulmonary artery pressure, and progressive clearing of alveolar oedema. Nifedipine offers a potential emergency treatment for HAPO when descent or evacuation is impossible and oxygen is not available. The findings also suggest that hypoxic pulmonary hypertension is essential in the pathogenesis of HAPO.