Pulmonary hypertension (PH) is mean pulmonary arterial pressure above 25 mmHg at rest. Although considered a rare disease, the prevalence of PH in certain risk groups is higher, from 0.5% in patients with HIV infection to 30% in congenital heart disease (CHD) associating PH. In pulmonary arterial hypertension (PAH) associated with CHD, early changes of the vascular bed are reversible after correction of the defect, but there is a point from where the disease becomes irreversible or progression continues despite correction. Among patients with "operable" and "inoperable" CHD, there is a "gray area" group in between, which is defined by pulmonary vascular resistance equal to 4-8 WU/m and ratio of pulmonary vascular resistance to systemic vascular resistance of 0.3-0.5, measured by cardiac catheterization. In this situation a pulmonary vasoreactivity test is indicated. Pulmonary hypertension is a severe disease with increased morbidity and mortality. Pulmonary hypertension can result in death by decreased cardiac output, and heart failure.