Pulmonary arterial hypertension( PAH) is defined as a group of diseases characterized by a progressive increase in pulmonary vascular resistance leading to right ventricular failure and premature death. Down syndrome patients are characterized by early progression of PAH because of insufficient thickness of the pulmonary arterial media. In addition, associated left to right shunt lesions produce pulmonary vascular obstructive disease. There is increasing evidence for beneficial effects of the phosphodiesterase type 5 inhibitor in PAH. A 6-month-old female presented with dyspnea and failure to thrive(body weight 3.6Kg, 3rd percentile). Her symptoms deteriorated progressively in the 4 months prior to referral to the Chonbuk National University Hospital. A chromosomal study revealed 21-trisomy, and echocardiograms demonstrated a moderated sized atrial septal defect(ASD, secundum) and patent ductus arteriosus( PDA) with bidirectional shunting associated with severe PAH. Saturation of peripheral oxygen (SpO2) was slightly lower in the leg than that in the right upper arm. The cardiac catheterization revealed severe pulmonary hypertension(75/32/54 mmHg in the MPA). With the 100% oxygen inhalation for 10 min. the elevated pressures in the MPA were decreased slightly (73/22/46 mmHg). After the surgical ligation and division of the PDA, she was started on mirodenafil (Mvix, SK chemical. 5mg tid). At 3 months, the infant felt symptomatically better progressively, and improved evidences of the PAH were shown at the repeat catheterization (58/21/41 mmHg in the MPA, Qp:Qs=1.7:1) and echocardiographic study. The ASD was repaired by a pericardial patch under cardiopulmonary bypass. It appears that phosphodiesterase type 5 inhibitor (mirodenafil) can be used safely without significant side effects in the intermediate term in a PAH associated with congenital heart diseases in Down syndrome.