Chronic hypoxia caused by migration of native sea-level dwellers to high altitude or chronic lung disease leads to the development of increased pulmonary vascular resistance and pulmonary hypertension. This altitude-induced hypertension offers no obvious benefit and may indeed be maladaptive. A major mechanism thought to contribute to the development of pulmonary hypertension is hypoxia-induced loss of small blood vessels, sometimes termed rarefaction or pruning. More recent evidence caused us to question this widely accepted concept including the potent angiogenic effect of chronic hypoxia in all other vascular beds and the demonstration that new vessels can form in the pulmonary circulation when stimulated by chronic infection and lung resection. We tested the hypothesis that chronic environmental hypoxia causes angiogenesis in the adult pulmonary circulation by using stereological techniques combined with confocal microscopy to examine the resultant changes in pulmonary vascular structure in rats. We found that chronic hypoxia resulted in increased total pulmonary vessel length, volume, endothelial surface area and number of endothelial cells in vivo. This is the first reported demonstration of hypoxia-induced angiogenesis in the mature pulmonary circulation, a structural adaptation that may have important beneficial consequences for gas exchange. These findings imply that we must revise the widely accepted paradigm that hypoxia-induced loss of small vessels is a key structural change contributing to the development of pulmonary hypertension in high altitude adaptation and chronic lung disease.