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Introduction: Pulmonary arterial hypertension (PAH) is defined by a raised pressure in the pulmonary arterial circulation associated with small vessel narrowing due to proliferation of the endothelium and vascular smooth muscle. Idiopathic PAH should be distinguished from PAH associated with a causal disease. One familial type (familial PAH), gathered from one family, has recently been linked to a mutation of the BMPR 2 (bone morphogenetic protein receptor 2) gene. It seems important to compare the idiopathic form of PAH with these familial forms to confirm that the same diagnostic and therapeutic principles can be applied to familial PAH., Material and Methods: The demographic, clinical, haemodynamic and prognostic data from 34 cases of familial PAH were compared with those of 451 cases of idiopathic PAH. The genetic characteristics of the familial forms were also defined., Results: Familial PAH presented at a younger age than idiopathic PH (31 +/- 15 vs. 45 +/- 18 years p=0.002) without any other demographic difference (sex-ratio 2.09/1 et 1.42/1 p=NS). There was no difference in exercises tolerance (6 minute walking test 341 +/- 98 and 289 +/- 135 metres p=NS), in haemodynamic parameters (mean PAP 65 +/- 12 and 62 +/- 15 mmHg, p=NS), or in prognosis, with the exception of an absence of a vasodilator response in the familial group to nitric oxide challenge. We found the BMPR 2 gene mutation to be quantitatively and qualitatively comparable to previously published data., Conclusion: The only difference between these two forms of this illness were of a younger age at presentation and an absent vasodilator response in the familial PAH group. We do not propose that familial PAH should be treated any differently from the idiopathic form. Genetic counselling will need to be developed in line with the progress being made in the understanding of this condition.