Lung hypoplasia (LH) is a life-threatening congenital abnormality with various causes. It involves vascular bed underdevelopment with abnormal arterial muscularization leading to pulmonary hypertension. Because underlying molecular changes are imperfectly known and sometimes controversial, we determined key factors of angiogenesis along intrauterine development, focusing at the angiopoietin (ANG)/Tie-2 system. Lung specimens from medical terminations of pregnancy (9-37 wk) were used, including LH due to congenital diaphragmatic hernia (CDH) or other causes, and nonpulmonary disease samples were used as controls. ELISA determination indicated little ANG-1 change during pregnancy and no effect of LH, whereas Tie-2 declined similarly between 9 and 37 wk in LH and controls. By contrast, ANG-2 markedly increased in LH from 24 wk, whereas it remained stable in controls. Because VEGF increased also, this was interpreted as an attempt to overcome vascular underdevelopment. Hypothesizing that its inefficiency might be due to impaired downstream mechanism, endothelial nitric oxide synthase (eNOS) was determined by semiquantitative Western blot and found to be reduced by approximately 75%, mostly in the instance of CDH. In conclusion, angiogenesis remains defective in hypoplastic lungs despite reactive enhancement of VEGF and ANG-2 production, which could be due, at least in part, to insufficient eNOS expression.