A double blind randomised placebo controlled crossover trial was carried out to study the efficacy and safety of oral tadalafil. Eligible patients were randomised to receive either oral tadalafil or matching placebo after baseline assessment of WHO functional class, exercise capacity by 6MWT and hemodynamic study by cardiac catheterization. Randomisation, blinding and drug/placebo administration was done by two pharmacists of the hospital. Patients received tadalafil 40mg once daily or matching placebo for 6 weeks which was followed by a 2 week washout before crossing over to the other drug for another 6 weeks. Routine medications for PAH like digoxin and diuretics was continued through out the study. Compliance was assessed by the pill count method at 3 weekly intervals. Safety of the drug or placebo was assessed by noting adverse effects, vital signs and (SaO2) by pulse oximetry at 3 week intervals. Clinical assessments (WHO functional class), 6 MWT and hemodynamic parameters by cardiac catheterization were reassessed after 6 weeks and again at the end of the study.