Despite the introduction of new drugs that have changed the course of pulmonary arterial hypertension (PAH), some patients are still refractory to treatment and deteriorate rapidly. Long-acting phosphodiesterase-5 inhibitors are a new class of drugs that are effective in PAH. This prospective study assessed the potential of combination therapy with prostacyclin and tadalafil for treatment of severe PAH. We report four cases of severe PAH that deteriorated despite prostacyclin therapy. Two patients had Eisenmenger syndrome, one had pulmonary hypertension associated with scleroderma and one had histiocytosis X. All were treated with tadalafil, 10-20 mg once daily, in addition to prostacyclin. After 3 months of treatment, all patients improved clinically, with an increase in mean 6MWD from 214 to 272 m. In three patients, the New York Heart Association functional class decreased from IV to III. Echocardiograms showed no significant changes in pulmonary arterial pressure. Although this study was limited by the small sample size, it suggests that tadalafil in combination with prostacyclin is an effective treatment for severe PAH. Tadalafil may be beneficial for the treatment of patients with advanced disease.