Patients with congenital diaphragmatic hernia (CDH) suffer from severe pulmonary hypertension due to altered development of the pulmonary vasculature, which is often resistant to vasodilator therapy. Current treatment starts postnatally even though significant differences in the pulmonary vasculature are already present early during pregnancy. We examined the effects of prenatal treatment with the phosphodiesterase-5 inhibitor sildenafil on pulmonary vascular development in experimental CDH starting at a clinical relevant time. The well-established, nitrofen induced CDH rodent model was treated daily with 100 mg/kg sildenafil from day 17.5 until day 20.5 of gestation (E17.5-20.5). Importantly, this timing perfectly corresponds to the developmental stage of the lung at 20 weeks of human gestation, when CDH is detectable by 2D-ultrasonography and/or MRI. At E21.5 pups were delivered by caesarean section and euthanized by lethal injection of pentobarbital. The lungs were isolated and subsequently analyzed using immunostaining, real-time PCR and volume measurements. Prenatal treatment with sildenafil improved lung morphology and attenuated vascular remodeling with reduced muscularization of the smaller vessels. Pulmonary vascular volume was not affected by sildenafil treatment. We show that prenatal treatment with sildenafil within a clinically relevant period improves pulmonary vascular development in an experimental CDH model. This may have important implications for the management of this disease and related pulmonary vascular diseases in human.