The role of the cardiac catheterization for diagnosis and treatment of pulmonary hypertension (PH) is very important. When mean pulmonary artery pressure increased more than 25 mmHg, then PH is defined. But this is measured accurately only by the catheterization. And we can discriminate the etiology of PH clearly by pulmonary capillary wedge pressure (Ppcw) or intra-cardiac shunt (L to R) by blood oxygen saturation step-up, and both parameters are obtained by this method. The etiology of PH is diagnosed as left sided heart failure, if Ppcw is increased more than 13 mmHg. PH is produced by congenital heart disease (ASD, VSD, PDA etc.), when the oxygen saturation step-up is recognized. And PH is induced by any pulmonary disease or pulmonary thrombo-embolism or collagen disease or liver cirrhosis or PPH, if Ppcw is normal and no oxygen step-up is recognized.