A preliminary observational study by the investigators has shown that tadalafil, a selective phosphodiesterase-5 inhibitor (PDE-5) decreases pulmonary vascular resistance(PVR) in patients of eisenmenger syndrome (ES) resulting in increase in pulmonary blood flow (Qp), systemic oxygen saturation (SaO2), functional class and exercise capacity. The aim of this placebo controlled trial was to assess the effect of the drug on exercise capacity and functional class compared to placebo.