BACKGROUND There is increasing evidence for beneficial effects of the type 5 phosphodiesterase inhibitor, sildenafil, in idiopathic pulmonary arterial hypertension (PAH). The effects of sildenafil in severe PAH associated with an atrial septal defect (ASD) have not been clearly delineated. METHODS This extended case series reviews our experience with sildenafil treatment in three patients with severe PAH and Eisenmenger syndrome associated with an ASD. Case notes were reviewed for baseline and follow-up anatomic, clinical and haemodynamic characteristics. RESULTS Of three patients identified, median age 44.3 years (range, 28 to 59 years), two had large secundum ASDs and the other a sinus venous defect. All patients had severe PAH and were desaturated at rest and/or during exercise. Sildenafil was started because of progressive and severe effort intolerance. All patients experienced symptomatic improvement, had higher arterial saturations (range 8-19%) and improvement in effort tolerance (88 m and 56 m improvement in 6-min walk test distance (MWTD) in two patients). Right ventricular (RV) function and Doppler derived RV systolic pressure improved in two patients. Pulmonary arterial pressures decreased in two patients who had cardiac catheterization (range 4 to 14 mm Hg). Pulmonary vascular resistance decreased from 7.58 to 3.8 Wood's units in one patient who is now awaiting surgery. Another patient developed significant pulmonary vasoreactivity (8.3 to 6.2 Wood's units with 100% oxygen) after 16 months of sildenafil therapy. CONCLUSIONS Sildenafil not only relieves symptoms associated with severe PAH in patients with large ASDs, but also improves pulmonary arterial hemodynamics and RV function.