BACKGROUND Sildenafil (phosphodiesterase type 5 inhibitor) has been shown to be effective in pulmonary arterial hypertension (PAH). We evaluated the efficacy and safety of oral sildenafil in patients of severe PAH with special emphasis on dose response relationship, time of onset of clinical response and its effects on different haemodynamic parameters. METHODS Forty-four patients of severe PAH of either idiopathic pulmonary arterial hypertension [23 (51.7%)] or Eisenmenger syndrome [21 (48.3%)] were studied. All patients underwent six-minute walk test (SMWT) and echocardiography, while some also underwent cardiac catheterization. Sildenafil was started after a test dose and was gradually increased up to a target dose of 300 mg/day. Patients were followed-up 2 weekly for 10 weeks and monthly thereafter for functional class assessment and SMWT. Echocardiography and cardiac catheterization were repeated after at least 1 month of achieving maximal sildenafil dose (target dose or maximally tolerated dose). Drug safety and tolerability were assessed by monitoring patients for adverse effects including fundus examination. RESULTS Mean follow-up duration was 18.7+/-8.8 months (range 7-30 months). Mean maximum dose achieved was 276.1+/-62.2 mg/day (range 75-300 mg/day). A significant improvement in NYHA class (2.54+/-0.5 vs. 1.31+/-0.4, p=0.0001) and in SMWT distance (247.4+/-74.7 vs. 366.3+/-93.8 m, p=0.0001) was noted. All patients reported "feeling better" within 2 weeks of starting 12.5 mg thrice a day sildenafil. Marked improvement was noticed at 150 mg/day dose. Some minor additional benefit was noticed with further increase in the dose up to 225 mg/day. No further benefit was noted in improvement of NYHA class and SMWT distance by further increasing the dose of sildenafil. Haemoptysis as well as chest pain, if present, were also improved. On follow-up cardiac catheterization, a significant reduction in mean pulmonary arterial pressure (from 67.0+/-10.2 to 56.9+/-9.5 mm Hg, p=0.001), PVRI (from 19.5+/-7.0 to 11.1+/-6.9 WU m2, p=0.0001) and PVR/SVR ratio (0.6+/-0.3 vs. 0.4+/-0.2, p=0.013) with increase in cardiac index (2.9+/-1.1 l/min vs. 3.7+/-1.1 l/min, p=0.008) was noted. Systemic as well as pulmonary arterial oxygen saturations also improved significantly. Sildenafil was generally well tolerated, except for rhinorrhoea in 2, bodyache in 1 and headache in 1 patient. No visual symptom or change in fundus examination was noted. CONCLUSIONS Oral sildenafil improves functional capacity, haemodynamic parameters and is safe in patients with severe PAH. Benefits start as early as 2 weeks. The effects are dose related. A target dose of 150 mg/day appears to be optimal. Being very effective, widely available, relatively inexpensive, and very easy to use and very well tolerated without any major side effect, sildenafil may qualify as a first line medication for these patients.