Exposure to hypoxia leads to the development of pulmonary hypertension (PH) as a consequence of pulmonary smooth mus-cle hyperplasia. Hypoxia concomitantly stimulates lung expres-sion of angiogenic factors. To investigate the role of angiogenesis processes in development of hypoxic PH, we examined the effects of lung overexpression of angiostatin, an angiogenesis inhibitor, on development of hypoxic PH and lung endothelial cell (EC) density. Angiostatin delivery was achieved by a defective adeno-virus expressing a secretable angiostatin K3 molcule driven by the cytomegalovirus promoter (Ad.K3). Comparison was made with a control vector containing no gene in the expression cassette (Ad.CO1). Treatment with Ad.K3 (300 plaque-forming units [pfu]/cell) inhibited cultured human pulmonary artery EC migration by 100% and proliferation by 50%, but was without effects on human pulmonary artery smooth muscle cells. After intratracheal administration of Ad.K3 (10 9 pfu) to mice, angios-tatin protein became detectable in bronchoalveolar lavage fluid. Mice pretreated with Ad.K3 1 d before a 2-wk exposure to hypoxia (10% O 2) showed more severe pulmonary hyperten-sion than Ad.CO1-pretreated controls, as assessed by higher right ventricular systolic pressure (36.5 Ϯ 2.4 versus 30.2 Ϯ 1.4, respectively), aggravation of right ventricular hypertrophy (P Ͻ 0.05), and muscularization of distal vessels (P Ͻ 0.01). Lung factor VIII, CD31 immunostaining, as well as eNOS expres-sion were significantly increased after exposure to hypoxia in Ad.CO1-pretreated controls, but decreased in both normoxic and hypoxic animals after treatment with Ad.K3. The results show that inhibition of hypoxia-induced stimulation of lung angiogenic processes aggravates development of hypoxic PH. This suggests that endogenous lung angiogenesis counteracts development of hypoxic PH.