Pulmonary arterial hypertension is characterised by the presence of pulmonary hypertension (mean pulmonary artery pressure >25 mmHg at rest or >30 mmHg during exercise ) and normal pulmonary wedge pressure (<12 mmHg). Several risk factors for pulmonary arterial hypertension have been described. In the absence of any factor or condition suspected to play a causal or facilitating role in the process, pulmonary hypertension is "unexplained" (primary pulmonary hypertension, PPH). PPH is a rare condition, with an estimated incidence of 2 per million people. Recent genetic studies have identified mutations in the bone morphogenetic protein receptor-II (BMPR-II) gene, a receptor member of the transforming growth factor-beta family, in a majority of familial cases of PPH. Interestingly, 25% of patients displaying sporadic PPH may also have mutations in the BMPR-II gene, emphasising the relevance of genetic susceptibility for this severe condition. Other molecular and biochemical processes behind the complex vascular changes associated with pulmonary arterial hypertension are currently investigated. Type 1a glycogen storage disease caused by a deficiency of glucose-6-phosphatase has an estimated incidence of 1 per 100000 with a few reported cases of unexplained severe pulmonary hypertension. The occurrence of pulmonary arterial hypertension in type 1a glycogen storage disease could be due to vasoconstrictive amines such as serotonin, a pulmonary vasoconstrictor and growth factor for vascular smooth muscle cells stored in platelets.