Primary pulmonary hypertension (PPH) is a serious pulmonary vascular disease occurring mostly in adult women. Although its occurrence in families was reported within a few years after the original clinical report, PPH was formerly believed rarely to have a genetic basis. Recent progress has not only clarified a basic molecular mechanism for PPH in families, but has also identified mutations of the same gene in many sporadic PPH patients, suggesting that its basis is commonly genetic. Extensive investigations in many centers are now in progress to provide a complete dissection of all the pathogenetic mechanisms of PPH.