Primary pulmonary hypertension (PPH) is a progressive disease with a poor prognosis. It is characterised by an elevated pulmonary artery pressure and pulmonary vascular resistance that ultimately lead to right ventricular failure and death. PPH is a relatively rare and neglected disease which, until recently, had been poorly understood and had no effective form of therapy. This, however, is changing with the rapid accumulation of knowledge relating to the disease and its management. In this article, we review the possible mechanisms that may have a pivotal role in the development of the disease.