The author discusses factors promoting the occurrence of acute mountain sickness and high altitude pulmonary edema. The level of altitude as well as the speed of ascent are important determinants and can be influenced by behaviour. A low hypoxic ventilatory drive presents a constitutional factor predisposing to acute mountain sickness and high altitude pulmonary edema. Individuals susceptible to high altitude pulmonary edema also show increased hypoxia vasoconstriction of pulmonary arterioles. The importance of an exaggerated hypoxic pulmonary vascular response for the pathogenesis of high altitude pulmonary edema is demonstrated by the observation that this illness can be treated or prevented by lowering pulmonary artery pressure with nifedipine. In most cases, however, acute mountain sickness and high altitude pulmonary edema can be prevented without the help of drugs, by adjusting the speed of ascent to the degree of susceptibility to these illnesses.