Introduction: Pulmonary arterial hypertension, both primary and secondary, continues to pose a therapeutic problem. In this study, we evaluate the efficacy and safety of a low-dose of oral sildenafil in 10 patients with pulmonary arterial hypertension. Methods: We administered a single daily dose of 0.5 milligrams per kilogram of sildenafil for 3 months to 10 patients with pulmonary arterial hypertension. Their average age was 26.8 years. Diagnoses were primary pulmonary arterial hypertension in 3 patients, and secondary pulmonary arterial hypertension due to congenital cardiac disease in the remaining 7 patients. Outcome measures included the clinical state, the mean pulmonary arterial pressure, and the indexed pulmonary vascular resistance; the latter two assessed at the beginning and at the end of the treatment period by cardiac catheterization. We also analysed the cost of the treatment. Results: Oral treatment was well tolerated, and resulted in an improvement of the functional capacity in 9 of the 10 patients. Pulmonary arterial pressure decreased from 70 to 60 millimetres of mercury (p equal to 0.05), and indexed pulmonary vascular resistance decreased from 21.8 to 15.8 Wood units per square metre (p equal to 0.006). The mean cost per patient for 3 months on oral treatment with sildenafil was 120.99 American dollars. Conclusions: A low dose of 0.5 milligrams per kilogram per day of oral sildenafil, instead of 1 to 4 milligrams per kilogram per day, provided early clinical and haemodynamic improvements, and proved less expensive. Additional experience is now required to define more reliably the true long-term benefits of this therapy.