Nitric oxide is a gas and free radical, which modulates pulmonary and vascular tone. Pulmonary vascular endothelial cell produce the nitric oxide. To define the relation between nitric oxide and hemodynamic parameters in children with pulmonary hypertension, we measured the nitric oxide concentrations of the right atrium, right ventricle, pulmonary artery, left ventricle and aorta in 40 patients during cardiac catheterizations. Patients were divided into two groups according to their pulmonary arterial pressure. In group I, the mean pulmonary arterial pressure was higher than 25 mmHg and in group II, lower than 25 mmHg. Pulmonary nitric oxide level in group I was significantly lower than group II (P<0.05). The right ventricle and mean pulmonary arterial pressures, pulmonary vascular resistance and pulmonary flow/systemic flow of the patients in group I were significantly higher than those of group II (P<0.05). In conclusion, we found low nitric oxide levels in patients with pulmonary hypertension and congenital heart defects.