Children permanently exposed to hypoxia at altitudes of > 3000 m above sea level show a phenotypical form of adaptation. Under these environmental conditions, oxygen uptake in the lungs is enhanced by increases in ventilation, lung compliance, and pulmonary diffusion. Lung and thorax volumes in children growing up at high altitude are increased. The haemoglobin concentration in highlanders is elevated. With respect to the decreased arterial oxygen tension at high altitude, this seems a useful adaptation. Blood viscosity also increases as a result of the increase in red blood cell concentrations however, and this has potentially negative effects on the microcirculation in the tissues. The decreased partial pressure of oxygen in the lungs of highland children is associated with a higher pulmonary artery pressure. Pulmonary hypertension, high altitude pulmonary oedema, and chronic mountain sickness form part of the pathophysiology afflicting highland dwellers. Birth weight at high altitude is decreased. Decreased postnatal growth has been widely reported in populations at high altitude, particularly in early studies from the Andes. Recent studies taking into account the effects of socio-economic deprivation, suggest that long-term exposure to altitudes of 2500-3900 m is associated with a moderate reduction in linear growth in children.