Primary pulmonary hypertension (PPH) is a rare, progressive and usually fatal disease. Its pathogenesis remains obscure. Probably multiple genetic and environmental factors play a role in the occurrence of PPH. PPH sometimes runs in families. We have studied two families in which some members were affected: in family A two sibs, their father and probably their paternal grandmother, in family B two sibs. When PPH is present in more than one generation (family A), an autosomal dominant mode of inheritance would be likely, whereas, when the disease is present in only one generation (family B), an autosomal recessive mode of inheritance is more probable. Treatment of PPH is based on pulmonary vasodilator drugs, because one of the most important mechanisms is thought to be an inappropriate vasoconstriction. Some drugs like oxygen, tolazoline, acetylcholine, isoproterenol seem to get poor results, while phentolamine, nifedipine, hydralazine and diazoxide show promise. Diazoxide appears to have some advantages because it reduces not only pulmonary arteriolar resistance, but also pulmonary artery pressure, without producing tachycardia.